The Colt Canada military rifles use the Canadian Weaver accessory rail uppers, but their LE guns use the standard MIL-STD-1913 rail.
Even before Colt Defense absorbed Diemaco, they would sometimes use Diemaco components for U.S. rifles, military and commercial, distinguished by the Devtek or "stylized D" marking. They were just an OEM supplier.
IIRC, they have also been soaking up excess production of Colt LE rifles - built to the exact same specs as U.S. market rifles, but since they are being shipped to an LE agency, they can be imported "assault weapons" without any of the usual rigamarole. This allows Colt to sell more U.S. produced weapons on the open market without importation issues, while filling large LEO orders. That being said, I couldn't say whether only some components are made in Canada and finished in the U.S., or completely finished in Canada, and simply shipped - or if Colt Defense sends them furniture and some small parts...
Then, if the LE agencies decide to trade their older rifles in for newer models, they can become available to the U.S. market. I think technically it could be argued that a "made in Canada" could be subject to 922r. once released on to the open market, but then again - how do you prove a set of handguards, an A2 muzzle device, ect. were made in the U.S. or Canada?
I am not aware of an Diemaco marked lowers in the U.S. though I suppose it's not impossible. Probably less than a half-dozen on the open market, if any. I do know that SGMLee of SHOT Show photo fame has or had a Deimaco barrel.
TacticalConcepts, one of our Canuk brothers is one of the SMEs on the Diemaco/Colt Canada rifles, and KevinB was CANFOR before being "the KAC guy."
In the end, though - the an LE/AR6520 is still "just" an LE/AR6520, that happens to have been made in Canada. Functionally and practically, it's no different, however, there are some that would probably place a little bit of extra value in the "Made in Canada" rollmarking.
I think I passed on one in '09 because it was overpriced for a '6520. Granted, the shop trying to sell it was well known for their atrociously high pricing - but I couldn't see myself paying more than a couple bucks extra for the "cool" marking.
Then again, MagPul lowers have sold for more than complete OBRs in the past, so there are people that'll do it...
~Augee