Nr. 4 Mk 1 Barrel Markings - Need help

[LA Gman]

I recently inherited a Nr. 4 Mk 1 after my Father-In-Laws passing and have been researching as much as I can about it. Unfortunately I do not know the story of how he came to posses it.
I am pretty sure by the markings that it is a 1944 ROF Fazakerley rifle and the markings on the stock under body forward match the rear band (MB21601).
What I can't find are any reference to "Barrel Markings. I have attached a photo of the barrel tip and those markings stamped into it.

Any information would be greatly appreciated.
Thank you,

Information

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