Question on a SG serial overrun carbine.

I just picked up a Inland which has the X suffix and is a duplicate serial number. It does have a curious mark on it.

On the front receiver ring just after the M1 mark there is a period. The line reads like this.

http://i1338.photobucket.com/albums/...pscab4ce0b.jpg

Anybody have any idea why this period is there?

Thanks in advance
Bob C.

Mine has the same marking. There are several variations of this marking depending on manufacturer. CAL.30 M1, CAL. .30 M1, CAL .30 M1 and CAL .30 M1. for example.