Like bsbg said...nothing is certain about Colt.
Colt did use up the last of the large pin receivers and then switched to the standard size during the production of the preban R6601.
So a couple of generalities can be made. If the lower receiver is of the slick side, ie. it has no mag. release fence, then it will be a large pivot pin receiver. It is generally conceded Colt never released a slick sided lower with the standard sized pivot for the semiauto rifles.
Now if the lower has the fence then it is possible the pivot pin could be the larger or the standard size.
If the hammer and trigger pins are the larger size then the pivot pin will be the standard size. Standard hammer/trigger pin is ~.165" and the larger size is ~.172".
Colt let a very, very few receivers slip out with the standard size pivot pin and standard sized hammer/trigger pins.
If you have the receiver in your possession there is an easy way to tell the size of your pin. The standard pivot pin diameter is .250". Take your pivot pin out and compare it to the rear takedown pin. If they are the same size you have a standard sized pin receiver. If the front pin is noticeably larger and won't fit into the hole the rear pin fits into, then you have a large pivot pin receiver.
And welcome aboard.