Les - Our posts overlapped!

You mention .429" Is this bore diameter? The 64 stamped next to the proofs will be bore size, Could not find my chart so I calculated it at .417"

How long is the case? More likely it is the .400 2 1/4" Kynoch, not the Purdey, but if it is long enough the .400 Boxer 3 1/4" has quite a wide variation in measurements that are within yours

If it is a side lever it will almost certainly have been made in the Birmingham Trade and retailed by your maker/retailer, ALL gunmakers did this including Purdey and H&H!

What makes you think it is early? Please post pics if you can, we will all be appreciative!

TH44