I checked biggerhammer.net but was unable to identify which stock I have.

It's an alloy telestock on a dark anodized 4 position extension (the buffer itself is unmarked as well and is dark colored).  The stock itself is simply marked with a "3" on the ejection port side near the adjustment lever.  It also has the old school pre-castle style nut on it.

It's mounted on a used Colt I just bought but who's to say it's "original" or not (as if it matters).  I'm just curious is all.

Also, have milspec buffer tubes always been milspec?  If not when were they standardized?  I noticed my Vltor stock won't fit on this tube is the reason for my question.

not sure about your stock... but if a Vltor will not fit on it, then it is an aftermarket unit.

Yes, milspec has always been milspec... Drasen changed the size of the tube when he designed the aftermarket clone of the military stock -- his design became sort of the de facto standard for all of the cheaper aftermarket parts, but unfortunately, there is no real aftermarket standard and they do very a little.

Oh, the reason for the size change was mostly manufacture technique.