You need to take it out of the holder and photograph it carefully.
Before you do, have a look at some more common dates of proof farthings to educate yourself about their appearance. Pre decimal Brit Proof coins do not exhibit the ultra cameo appearance that modern proof collector coins have. An example of a common date proof farthing would be 1951. I have a proof 1951 half penny which I obtained for about $10.
The reason I mention all of this is because if that coin is a genuine proof farthing of 1943, it has a catalogue value of about 300 pounds!