D&T: How about that. Looked under the top strap and there are three tapped holes under the rear sight. Good to know, though I'll probably not every mount a scope on it.
AimSmall was asking about value. One way, of course is to check auction sights for the final bids. Rock Island Auction has an upcoming one in February with several 629's. Problem is they're in multi-gun lots so assigning value to each gun is problematic. One lot has a 10-5/8" 29-3 and a 8-3/8" 629.
Two Smith & Wesson Double Action Revolvers -A) S&W Model 29-3 "The Silhouette" Revolver (The 629 doesn't have an underlug and has a non-removable stainless front sight.) They are estimating $1,400 to $2,000 for the pair. Splitting the value 50/50 between the guns, then that's $700 to $1,000 each (PLUS 18% auction fee and shipping). Monitor the auction to see where it actually closes. That will give an idea of value.