I think this is the type of trigger guard your rifle needs:

https://i.imgur.com/UpJljzA.jpg

https://i.imgur.com/tvB5tMn.jpg

https://i.imgur.com/9FvuGb6.jpg

Unfortunately this one is off my A series and not for sale. The only spare trigger guard I have is the tang type which was fitted to some standards and club standards in the 1920s.

Sorry, but I am always buying bits and bobs for these old BSA's. If I ever come across one, and you have not sourced one, ill get in touch.