Sorry- but that is not enough information to answer. Serial numbers are NOT unique to one maker. Post again with ALL information marked on the gun, and we will try to help.
Most likely ROHM or RG.
Herbert Schmidt Company.
Could have been Erma, Rohm, RG, Sauer or any of a half dozen others.
The caliber should be marked on the slide.
I have one that was pre war, mint condition, fired only once, made by Heckler.. Richard
It will usually be marked somewhere on the barrel.
It should be marked on the barrel.
Most likely a ROHM
Should be marked on the weapon somewhere.
Impossible to tell from just the sn. if you found the serial number, you will be able to find the caliber of the pistol somewhere carved in the metal.
Try Blue Book of Gun Values