S&W made a number of runs of 629s with 7.5" barrels, usually special orders for their big jobbers.
The 629-2 went from 1988 to 1990, and had a transitional step in 1989.
The 629-3, "The Magna Classic Stainless" was a run of 1,200 made in 1990.
Tell me this, Son...
Does it have:
1. An unfluted cylinder?
2. Counterbores on the cylinder?
3. Non S&W typical wood grips?
If so, then you have one of 750 guns made in June of 1989 for Accusport.
Retail on those was $650.