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I think HD means, on the left side of the receiver that is stamped 03-A3, one of the arsenals stamped "/4" on top of the 3 in A3.

Neal

https://www.milsurps.com/images/imported/2012/02/1903A4_8-1.jpg
https://www.milsurps.com/images/imported/2014/05/1903A4_7-1.jpg

That's a really nice rebuild, fresh from the arsenal. I would guess it is a "Condition A" rifle sold by CMP in their 1999 lottery. If so, they will be able to send you a letter to that effect for a...