when did Light heavyweight first become an exclusive division?

Bob Foster fought at heavyweight quite a bit.

I don't recall Marvin Johnson or Matthew Saad Muhammad fighting anywhere but light heavyweight. If they did, it may have only been once. But I don't think either did.

Victor Galindez was a light heavy until his very last fight.

 

He certainly did, most famously against Frazier and Ali.

But he was the first great LHW who became exclusively known as just being a great LHW, but he still mingled.

I think the LHWs stopped mingling with the Heavies with the birth of the Cruiser Davison in 1980 and the end of Same Day Weigh Ins not long after. But even before that in the mid 70s, you had a rather unremarkable crop that stayed clear of the heavies.

 

Marvin Johnson.

 

I think that the money has always been bigger at heavyweight, so light heavies have generally tried some sort of foray up.