First, dragging the cylinder while checking the timing is NOT correct for a S&W revolver, in almost any case this will cause a failure of the lock engaging. These are Combat revolvers, not precision target guns. They are intended to function well even after many years of neglect. To that end, the lockwork is designed with a certain amount of slop to insure function when they are not well maintained. One of those features is that the hand will disengage from the ratchet before the stop locks in. Another feature is that the stop drops early in the rotation, which is why any S&W that has seen use will show a drag line. By design the S&W lockwork is designed to rely on the inertia of the cylinder to carry it completely into lock, it's not driven into lock because that can create a hand/lock bind and freeze the trigger completely. It's why the owners manual advises against staging the trigger. Yeah it can be re-timed a lot "tighter", however failure to keep the lockwork in tip top condition can result in problems with function. Basically, your method for checking the timing is wrong and I don't care what you've been told, it's still wrong.

That said, once locked the cylinder should not come out of lock during firing or with any amount of reasonable finger pressure. There has been a history of this problem with the 44 Magnums and it was addressed with the durability enhancements in the ? late 80's. As I understand it, the stop notches were extended and I suspect the stop spring was also made a bit stronger. I would advise that you compare the length of the stop notches in your 629-1 to those in your 629-4. If there is an obvious difference in length, give S&W a call and see about having your 629-1 "enhanced". At one point this was a warranty repair, today with your 629-1 being pre-1989 you may have to pay for it. I would also suggest that you check your 629-1 for End Shake, another common issues with the Magnums that have seen use, that won't be covered by warranty as it's considered normal wear. BTW, End Shake will aggravate this problem, so it's possibly a factor in your gun.

Basically, your 629 is due for a tune up and it's time for a trip home. Good news is that your's is so close to the cutoff for the lifetime warranty that S&W may do it on their dime, however you won't know that unless you call. Even if they won't pay to fix it, their pricing for these services is very reasonable and every report I've seen indicates top notch professional service.