UK 3d 1920

https://en.numista.com/catalogue/pieces21182.html
https://en.numista.com/catalogue/pieces2788.html

 Can someone explain if there is a physical difference between these two, for the year 1920 only please? I know the metal was silver (.925} then silver (.500) but I have one and do not know how to tell - whether it is KM# 813 or KM# 813a number. Is there anything, like lettering or where it points to (between beading), or head size, designer's initials location (like 1926 ME) etc to look out for? Thanks  

P.S. The other coin that also changed that year was the sixpence KM# 815 and KM# 815a - luckily the shilling and florin and half crown stopped in 1919 with silver (.925) then the next year was silver (.500).

I've found the same issue with the 1967 Canadain 10 and 25 cent peices.
There are both .500 and  .800 silver for each of them.
The books show them at the same weight (respective to face value).

I just gave up and listed them as .500, just to be on the safe side.