If you truly have a factory original, unlocked Blue Label lower with a mil-spec diameter front pivot pin, you have an object of value.
More likely, you are unsure of what to look for or a previous owner removed the sear block.
For comparative purposes, here is my Sporter Match HBAR with the sear block:
And here is my Sporter Lighweight that I removed the sear block from:
Notice the block is the piece marked "COLT" that fits inside the lower receiver just ahead of the rear take down pin.
As far as the size of the forward pivot pin, if you can even fit it into the rear take down pin hole, then you have a mil-spec size pivot pin. Sounds like you do indeed have a small hole pivot pin. Any standard upper receiver will fit without the use of adapters or conversion pins.