I believe one key difference between the round and rectagular E-codes is that the rectangular ones support remote beam adjustment while the round ones don't.

My neighbors when I was outside the US got a brand new Euro-spec 83-ish 300Dt back in... uhh... 83-ish. I remember going through it with a fine tooth comb comparing it with their US-spec 77 300D. Spotted things like the joggle in the wiper arms, deeper A-pilar channel, extra black strip in the C-pilar channel, slimmer front seatbacks, etc. I wouldn't put big money on it but I'm pretty sure it had round E-code lights.

Sixto
95 S420
87 300SDL
83 300SD