Barrel Weight Markings and Frame Size

[Dean Romig]

Yes, your gun is too early to be a 1/2 frame but several of the frame dimensions of a 1 Frame and a 1/2 Frame are the same. Yours is a 1 Frame without question.
The barrels may very easily have had a pre-struck weight of four pounds exactly and the 4 appears to have been stamped in the right location for the weight.
The 13 stamp is puzzling. The gauge is sometimes stamped here on some of the early lifters.... hmmm...

Coincidentally, 11009 is the part number for the pilot bearing in the rear of the crankshaft of all of the Volvo four-cylinder engines fitted with manual transmissions up to the early '80's when it was replaced by the 181393 sealed bearing. (useless bit of Volvo knowledge bouncing around the confines of my cranium)