No.4 Mk1 markings for survey

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I’m currently surveying my No.4 Mk1 (T) trials rifle. I could really use some help with understanding a few of the markings on the receiver and barrel. I have attached some photos.
- Why so many repeat crown inspection marks on barrel? 5 plus the GR proof.
- AA 16 on receiver?
- B6 on barrel?
Apologies for the lousy dusty photos. I’ll take high quality photos of the rifle for future posting.
Any assistance that can be offered would be much appreciated.
Thank you.