Hi Tony, your post raises several interesting issues. I think it is generally understood that the coin you refer to, the Jersey KM#20 Queen Elizabeth 1/12 shilling, was not struck in 1945 (obviously) and the date is commemorating the liberation of the island by the Allies at the end of WWII. This is backed up by the fact the strike is (correctly) identified in the Numista catalogue as ND (no date) with the mintage date shown afterwards in brackets because it doesn't appear on the coin.
However, when I searched the interweb to look for info on these coins, I actually stumbled across the previous issue, namely the KM#19 George VI 1/12 shilling which has the same reverse. When I looked further, I found that these coins weren't struck in 1945 either but in fact were struck over 3 years - first in 1949 then again in 1950 and 1952. This is where the Numista catalogue (and possibly Krause as well) appear to be incorrect.
After the king died in 1952 and his eldest daughter, Elizabeth, became queen it was decided the continue using the same reverse for the first coins minted under the reign of the new monarch. The Coinage Act of 1953 actually authorized this new series but the coins were not struck until 1954. It's also interesting to note that this same Act also authorized the Queen Elizabeth 1/24 shilling although none were ever issued.
Source:
http://jerseycoins.com
Just because you can't see it ... doesn't mean it isn't there - Anon.
Former coin and banknote catalogue referee.