If you mean the number stamped on the underside of the receiver; AFAWK that is not the serial number. The authorities on the subject of Rosses I have read say that the absence of a serial number on the barrel may indicate that the barrel was replaced, but considering the company was only around for about 20 years, how many barrels would they have needed to replace in that time? I'm guessing not too many as they they were chrome vanadium steel IIRC. That said, there were considerable stocks of parts left over when the company closed down and more than one gunsmith in Quebec was assembling rifles from parts into the 1930s and even later from one account. Serial numbers may have been optional, or the practice may have not been universal depending on who the buyer was and if they bought in bulk? We may never know now. I have at least one 1905 R with the SN only on the butt and buttplate, and they match; the usual markings on the barrel like yours, but no SN and no sign there ever was one.