I was recently asked about assist rates in the first years that assists were officially awarded. As you may know, the PCHA began awarding assists in 1912/13, and the NHA followed a season later. And as you might also know, for many years very few assists were awarded per goal (as compared to modern standards). How few? This few:
It may be worth noting that when the PCHA finally dropped the rover after the 1921/22 season (as they began playing an interlocking schedule with the WCHL, which did not use the seventh man), its assist rate dropped from 0.57 to 0.51, and stayed at that level or lower, whereas in the final seven seasons with the rover, the PCHA had an assist rate of between 0.57 and 0.60 six times. This suggests to me that at least part of the reason that the NHA/NHL had a lower assist per goal rate is the lack of a fourth forward.
The first really big jump in assist rates occurred in in 1929/30. In 1928/29 there were 0.60 assists per goal, but there were 0.82 the following season. This is clearly due to the change in the forward passing rules that season, which made individual rushes less important and passing plays more important. The rate went to 0.92, then 1.05 in the next two seasons, and would never drop below that level again.