Here's some pics:

Note area front latches mount to... the early one (left) has raised areas where the latches mount, and the late one (right) does not .

In the back, the early top has a smoother transition from the lingitudinal rib to the flat space where the rear latches mount. It's one smooth line from the top of the rib down to the flat area. Also, at the very back, the space behind the seal is hollow when looking from the inside.

Did you weigh them ?? I *think* one is lighter than the other.


B

On the late one, the rib slopes down a bit as it widens to the width of the rear latch mounting point, but then has a sudden step down to the level of the latch. At the rear the area behind the seal is solid instead of hollow.

There may be other differences, but these are the ones I noticed.

No, I didn't. The early one has to be lighter.. the late one is solid across the rear seal area, and is thicker across the front as you can see in the first pic. It looks like they just filled in the area between the raised parts to make it raised all the way across.

Anyone know what the year cutoff is between early and late? Or if they're interchangable between early and late cars?

yes they do interchange
also the cut off was mid 73