Mike
after Model 13 was introduced in 1974, marked as the Model 13-1. There never was a Model 13 with no dash number, because it already had the dash one features (left hand threaded extractor rod, 3 screw frame and heavy barrel) when it was first produced as the .357 Magnum version of the Model 10-6.

So, the order goes like this (by year of introduction):
13-1 - 1974
13-2 - 1977
13-3 - 1982
13-4 - 1988
13-5 - 1997
So if 13-6 was first then the 13-1?
Wat came after the 13-5?