Hi Gary,
That looks like a nice find.
I am no expert and information on the Internet is always confusing, so I guess you have come to the right place to become even more confused
I have an interest in WWII Aircraft Memorabilia and must say I'm confused? The A.M. (Air Ministry) Reference 6A/745 I think applies to the P4A compass, which I believe went into a number of aircraft, including Lancasters.
The top clearly says "P10" but there are only two parallel lines for heading, I think the P10 had either a cross or a Tee , which I believe was designed to prevent confusion between forward and backward headings when set in the heat of battle on the earlier models.
Regardless of which model you have, you are lucky to have picked up a piece of WWII History
With regards to the Radium, i think you will find arc5stinson.38188 is spot on. Google things like "British Radium Girls" and you will see why there is so much fear around these beautiful parts of history. I think the one shown in that post is actually a P8. I cant blow up the photo any more without losing detail, but it looks like a P8 on the ring. I recently purchased one of these and the Air Ministry Reference (AM) is 6A/0.726. This mode has "AFT marked on the housing, as shown in the post. The curved lever next to AFT allows you to unlock and turn the top part to set your bearing.
I picked up a P8 recently (Spitfire and Hurricane). Because I was concerned about it, I took it to a company that services avionics. They were pleased to view it, still in a box and in perfect working order. At no charge, they ran a radiation test instrument over it (dosimeter?) and concluded there was no problem. The highest emissions came from the top, so I do not sleep with it under my pillow. The tech advised me not to lick it! (See reference to British Radium girls)
They were servicing a P8 and let me see one in pieces. They then offered me three times what I paid for mine, as it was likely to be cheaper than the restoration they were doing.
When I said it was free of charge to have it checked, they are currently going through their shed to see what other WWII treasures they can sell me for my collection!
Please put up some more photos of your compass and let us know what your research reveals,
Cheers, Gary