The coinset for 1976 included this piece in ARM (the Spanish mint name name for "sort of proofish but not really proof", its a complicated and very controversial issue in Spanish numismatics). The problem is that, while the catalog entries for all the coins from the 1976, 1977 and 1979 coinsets distinguish between ARM (by calling it proof) and regular issues, it does not figure in the 50 cts from 1975 entry, and I can't edit it to add it, it redirects me here. If someone would be kind enough to do it, I would be grateful.
(https://en.numista.com/catalogue/pieces7576.html )