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The Numista catalog page for this coin series lists two mints for this coin: Brussels (Belgium) and Paris (France). The coins with the A mintmark are obviously from the Paris Mint, which would imply the two dates without an “A” mintmark were minted by the Brussels Mint. However, those coins have two privy marks (a caduceus and a sword) which would make them products of the Utrecht (Netherlands) mint. The SCWC indicates one or both of those dates was also struck by the Brussels mint until 1909. Were any of the coins with the Utrecht privy marks actually struck at Utrecht, or were they all struck at the Brussels Mint (until 1909) with only the dies being made at Utrecht? Should Utrecht be listed as a mint of these coins on the Numista catalog page?
The coins without the A mintmark for Paris were solely struck in Brussels. Even the dies were not made in Utrecht. In the following text below you can read how those Utrecht privy marks came on the coins (last picture). To answer your questions: ALL the coins till 1909 were struck in Brussels, and no, Utrecht should not be listed as mint of these coins since coins nor dies were made in Utrecht.
The following text comes from the book “Buitenlandse Muntslag in Brussel” ("Foreign coinage in Brussels") by Didier Vanoverbeek. Unfortunately, the text is in Dutch but I give a translation below the pictures.


Translation:
1. William III (1849-1890)
The first coins of the Grand Duchy of Luxembourg
Before the introduction of its own coins and bank notes, the currency symbols of the surrounding countries circulated on the territory of the Grand Duchy of Luxembourg. Although Luxembourg had become a member of the German Zollverein since 8 February 1842, the population continued to prefer Belgian and especially French currency. Because Luxembourg had been in a personal union with the Netherlands since 1839, guilders were also accepted in the State treasuries.
In order to bring some order to this monetary chaos, a number of foreign coins were decoined in the middle of the 19th century, and a start was made with a national currency circulation. The law of 20 December 1848 introduced the franc as the currency, and on 9 January 1852 a new law was promulgated providing for the issue of bronze coins of 2 1/2, 5 and 10 centimes, for a total of 150,000 francs (increased to 250,000 francs by the law of 1 December 1854).
The grand-ducal decree (ghb) of 1 March 1854 then laid down the modalities of issue of these coins. In the meantime, however, the issue of these coins had already progressed considerably, because on 23 July 1853 the contract had been signed between the administrator-general for Finance of the Grand Duchy, Norbert Metz, and the consul of the Grand Duchy in Amsterdam, Leo Lippmann, who undertook to produce these coins for 4.60 f/kg.
Lippmann then had the production in the “Munthof” in Brussels subcontracted to the firm Lassen & Cie. The Belgian Minister of Finance, Charles Liedts, gave his approval for this on 13 August 1853. The models and mother dies were also made in Brussels, i.e. in the workshop of Pierre Joseph Braemt, by the Luxembourg medallist François Barth-Wahl. The pieces were struck from January to May 1854.
Picture of the “Munthof”:

After these pieces were struck in Paris in 1855, 1860 and 1865, similar pieces were minted again in Brussels in 1870. The law of 1 August 1869 (ghb 20 October 1869) had decreed bronze coins for an amount of 150,000 francs, and at the opening of the tenders on 22 November 1869, Josse Allard, with 2.92 f/kg, had submitted the most favourable price. The lower price for carrying out this production compared to the price in 1854 can be explained by the fact that the blanks were this time made in France by the firm Oeschger & Mesdach de Terkiele from Biache St.Vaast and were therefore not included in the tender price.
Léopold Wiener made the work dies for 14 f/die pair for the 5 and 10 centime pieces, and for 12 f/die pair for the 2 1/2 centime pieces. Production lasted from February to April 1870.
François Barth-Wahl (Luxembourg 25/XI/1806 -ibid.8/III/1867)
Born as François Barth, but after marrying his sister-in-law he adopted a double name. Engraver based in the Grand’rue in Luxembourg, who designed and engraved the first Luxembourg coins as well as the first Luxembourg postage stamps (in 1852 ).
Technical features and overview of production:
10 cents 5 cents 2 1/2 cents
Diameter 30mm 25mm 20mm
Mass 10 g 5 g 2.5 g
Edge Smooth Smooth Smooth
Alloy Cu 95 Sn 4 Zn 1 Cu 95 Sn 4 Zn 1 Cu 95 Sn 4 Zn 1
Engraver F. Barth-Wahl F. Barth-Wahl F. Barth-Wahl
Mintage 1854 500,000 680,000 640,000
1870 1,313,050 303,900 209,880
Service life 1854 83,333 64,762 60,952
1870 35,040 22,679 28,898
Reference km·23|l·264|w·254 km·22|l·265|w·255 km·21|l·266|w·256
Description of the pieces

Obverse: The crowned coat of arms of the Grand Duchy of Luxembourg, surrounded by the inscription GRAND-DUCHE DE LUXEMBOURG
At the bottom, on either side of an asterisk, the mint master's mark of H.A. van den Wall Bake (a Roman sword) and the mint mark of the Mint of Utrecht (a Mercury staff).
Reverse: The value indication and the year, surrounded by a bound oak and laurel branch. At the bottom, the name of the engraver BARTH.
Incorrect mint marks?
The bronze coins minted in Brussels in 1854 and 1870 bear the Utrecht workshop mark, a mercury staff and the mint master mark of Mr. van den Wall Bake, a Roman sword. Because Luxembourg was under the guardianship of the Netherlands, it was initially assumed that the coins would be minted in Utrecht, and therefore the engraver placed these mint marks on the mother dies. During the production of the coins in Brussels, the mint marks were apparently not recognized as such and were therefore improperly used. The Dutch government did lodge an objection to this, but sixteen years later apparently no one remembered the incident and the same mint marks were placed on the coins again in 1870. The coins minted in Paris in the meantime do bear the correct mint marks, i.e. an anchor for the chief engraver Albert Desiré Barre and a bee or hand for the mint director.