Absolute Phrase/odd phrase--

"There were two gates, one at either end of the room."

I know this makes sense and is the probably the best way to write this. But can you explain this phrase structure to me:

S V C, noun + adjectivel phrase (prepositional phrase/participial phrase etc)

OR

S V O, noun + adjectival phrase





1) Is this an absolute phrase?

2) Is it common to write like this?

3) Is it reduced from one of these or is not reduced. Is it just another phrase type?

There were two gates, one was at either end of the room.

OR

There were two gates, one of which was at one end of the room, and the other of which was at the other end of the room.

OR

There were two gates. One was at one end of the room, and one was at the other end of the room.

Thanks