I believe the crossmembers or "arms" these pins were inserted into were of uniform thickness (see following photo of a surviving arm) although I have seen very few of them. The other arms (perhaps two or three) held heavy duty porcelain cleat insulators for the heavy DC copper cables to seat within (they had holes drilled through them for the cleats). At least one of these arms had holes in it for the referenced insulator pins too. However, I think the cleats were used with those since day one. Much of the elevated Orange Line employed huge white porcelain cleats. The earliest section (towards Boston, approximately a mile) employed the famed CD 267 and 267.5 glass insulators. Further out to the line's terminus a few miles to the west, cleats were used...and on the same kind of crossarms. I believe some of the broken glass cables were replaced by cleats through the years... instead of using another type of glass insulator to do the job.