New No.1 Mk4 owner

[JayDuff75]

Hello to all. I just picked up a No4 Mk 1 and was curious if anyone could help me out with the markings so I can learn of its history. I have decoded a few, but not all as I am new to this. Here are some pics of the markings.

Information

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[Surpmil]

It's a Maltby and looks to be dated 1942. I have doubts about the SN as it is a restrike of one of that appears party illegible. It could be the number began "BY----", but what may be a heavier restrike of the Maltby "M" was mistaken for an "H". You can see there is most of what is either an "8" or a "B" ahead of the "Y" in the upper serial number. "8Y---" would not fit the sequence I believe so it must be "BY----"

The bolt is obviously a mismatch, the barrel was replaced with one dated 1945. It's London Proof House marked and if that is a D in a circle on top of the Nock's Form (barrel flat over chamber) then Skennerton gives that a "draw-lapped barrel", which someone will have to explain!

There's a small circular mark on the barrel ring I cannot ID either.

[JayDuff75]

Thank you for the insight on the markings. I had it all broken down and had checked a few of the markings against threats on here. Just wanted a few sets of eyes to make sure I was reading things right. I reassembled it shortly after so i'll take a pic of it and post here in a bit.

[Peter Laidler]

Just a minor correction but the D within a circle indicates that the barrel was manufactured using a different procedural method in order to eliminate waste. This was introduced late war and is clearly indicate in your 1945 barrel