I'm looking at a Smith Model 60. It says "MOD. 60" on the frame but the Serial Number is R241XXX. The no dash 60 ended in 1971 and the 60-1 started in 1972. The Serial Number R241XXX would put it being manufactured 1979-1980. Is this a left over 60 no dash frame used in 1979-80 or is there some other explanation?