Sterling 7.62 conversion questions.

[Sapper740]

I'm considering buying a Sterling 7.62 NATO conversion but the U S stamped on the receiver ring concerns me. Lacking any Flaming Bomb ordnance stamps elsewhere my concern is the U S stamp could mean the rifle is "Unserviceable"? I'm waiting to hear back from the seller but he doesn't appear to be much of a Lee Enfield expert so my questions may not get answers. Would any of you Sterling conversion experts please take a look at my pics and offer your opinions?

Thanks, Derek