Just spitballing here but my 73 is grounded up front too and it's wiring and that could be common to ground the front as there's a lot of grounded things up there close to where the battery supposed to be?.
Yes plastic lamp fixtures and/or fixtures mounted in plastic must have a actual wire for chassis ground path. For example a 68 of 9 barracuda front park/turn fixture has a ground wire while 67 does not have.
Majority of rear lamp fixtures are metal and use that metal to metal contact for chassis ground path. So a plastic rear fixture, ( mounted in a bumper maybe? ) would have a ground wire somewhere. There could be a ground incorporated in those fixtures mounting to the bumper with or without additional ground wire from bumper to chassis. Whenever and wherever they found that connection through hardware wasn't a dependable path, they would add a ground jumper. The little jumper at the steering column mounting is a very good example of this. Without that little jumper the horn might blow and it might not. Column mounted lamps might work, might not.
Same condition could occur at bumper mounted lamp fixtures.
