Factory black channel is bevel cut on the corners as well. This one looks like the ends overlap or something. I'm not sure what's going on here. Could be factory, but
I have a set of NOS black channel. Grp 12.810, 20164509, four needed for the short sides, and 2016510, two needed for the long side. The parts book lists this from 1979 2nd design through partway through 86. They already come with a bevel cut, ready to attach. I'm assuming with some sort of butyl crap. Not sure what was going on for 1978 hatch roofs.
Parts books show there's a 1st design smaller glass T-top for 1978 and partway through 79. Then they went to the 2nd design in 79 which stayed that way through 1985 for the most part.
The whole channel set listed from GM for all 6 pieces $10.70 back in July of 1987.