I would'nt worry about having a part number as far as the part you need is concerned. Front pto/blower double u joints are either 13/15 splines, or 15/15 splines.
When you call Richard's, just tell them that you need a 13 spline/15 spline double u joint front axle pto shaft to snowblower connector. They will know what you mean.
As I said, they are good people, and they know Gravelys.