Chronographs weren't common in 1979, nor affordable, so I couldn't tell what the velocity was on the loads in question. I determined there was a problem from sticky bolt opening, a measurable difference in the expansion of the case just above the web, and a marked difference in accuracy. Once I worked the load up again to the same accuracy level as the older batches of that powder, I was exactly one grain below the other loads. Since the original load was 25.5 grains of powder, and the errant lot ended up at 24.5 grains, the percentage was enough to make me sit up and take notice. And yes, the primers "weren't happy".
It's much easier now to develop loads, since chronographs are quite affordable and common. I personally own the PACT Professional Chronograph, with printer, and find it a valuable tool.
Hope this helps.
Fred