The claws on the cocking arms can wear and that will allow them to pop out, I had one with the same problem.
If you've determined you do want to fix...
That's very close. Just taken two other measurements, the latch rod is 10mm in dia and it's 166mm from the front of the seal to the rear of the notch...
Don't remember the BSA family, but my teenaged self certainly did recall the comely young lady on the Milbro Cougar ad (come to think of it that may...
hahahahhah.... Ah you may jest. Its not for a B2, but for a Model 55, which the preceding and slightly better made break barrel form Shanghai. So not...