1972 Roll "cage" and Hardtop: what do I have?

[wcelliot]

I started a build thread but thought I'd also ask the question here:

I'm starting my modern drive-train build with what appears to be a very original 1972 Blazer... my question concerns the top and roll bar... what do I have?

It doesn't look quite like other double walled tops I've seen and I've never seen a roll bar like this... BUT the inner part of the top appears factory molded to fit around where the roll bar attaches at the windscreen header.

Is this factory/rare/desirable? (I had planned on no cage and keeping the top off most of the time in favor of a soft top). But if I have something really unusual I may keep it...

Thanks!