Welcome to the fascinating - and frustrating - world of Carcano collecting!
The FNA-B RA and RB series were made late in the war, and many were made up from parts laying around. Like barrels that had "passed the gauge" but had not been completely stamped, for example.
That's my guess for yours, and it's only a guess. Both the German and Italian Carcano books use the word "inconsistent" a lot. There is little chiseled in stone with this model. Generally speaking lack of the maker's mark means later production. The type of wood shown is also consistent with a late rifle - beech was getting very scarce in the final days.
I also believe it narrowly missed being converted to 7.9 caliber.
There is no set date of manufacture for the RA & RB series either... the way I read it, they were made between mid 1943 and early 1945.
Sorry I can't be more specific, but so much of this model is just not known. If you have the action from the stock sometime, look the bottom of the receiver ring over closely - sometimes there will be a date. The later it was made, the less likely it will be there, however.
Oddly, I have one a few thousand off that has the navy "crusader shield" and a tiny anchor.
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#123 South Main St
Haystack, Nebr
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We can't always answer all questions, but it's still a great site!!!
SW