Hi Les,

It sounds to me that you have got what they call an 'A' Series. These were the very last of the BSA Light Patterns (1930s in date I think). There will be people on here who know far more than me abut them but, as I understand it, the A series variant of the Light Pattern is a reasonably rare model.

I should add that they are a cracking gun - I have a soft spot for the light patterns!

Best

Olly