All mine are from the early to late 1970s.
My 1975 61/2" blue had iffy timing but was super accurate.

My 1976 83/8" that I used for IHMSA was very accurate but went out of time. I sent it to S&W and told them what it was used for.
Whatever they did worked great and I never had another problem after about 2,000 rounds of Sierra 240gr.jhp and 24 gr. of WW 296.

In 1977 29s were like hens teeth and a friendly dealer/distributor offered me a 61/2" blue. We opened the box and the bluing was awful as were the grips.
He got another out and it was the same. The third time was the charm - a beauty.

My 1980 m629 also went out of time and was returned. When i got it back the windage sight had to be moved all the way to the right.

The first 29s had a cylinder bolt that would back out and jam the gun, and the fix was reversing the thread which became 29-1.