I would guess its a marking from the country that captured it in WW1

 

I've seen the U about a half dozen times on Finn-used as well as non Finn m91s. Somewhere I have pictures saved. I'll go digging for them.

 

That'd be great, Racer... A closer look at the 5 seller pics indicates the rear sight has been renumbered and there are the typical Finn sling swivels. There's also an SA n the left side of the barrel. I've just never seen that long U on a Finned Mosin before.

 

One of these days I'll find an original Sesty that I can afford...

Or maybe not.

Greg