There are a few differences between the SP1 and the M16A1, The front take down pin on the SP1 is a large pin with a slotted screw head where the M16's had the small push pin like todays AR15's.
Also while the earliest M16's had no forward assist or fenced mag release the M16A1 did, The SP1 is more of a copy of the earliest M16 and had no mag fence or FA.
There are some transitional green label AR15's out there, box was marked R6500, receiver was marked Colt AR-15 A2 Govt Model or Sporter II with A2 pistol grip and buttstock assembly, the earliest uppers are the older M16A1 style with FA and no brass deflector, older windage site wheel, heavy profile A2 barrel with 1/7 twist, A2 birdcage and A2 handguard and new style delta ring.

Early run marked as SPORTER II with M16A1 style upper and SP1 lower:


Later run Marked A2 Govt Model with A2 style upper and SP1 lower (notice area just aft of the rear take down pin is old style M16a1 on both)


Late Govt models have the new A2 rear site, later ones have the bayonet lug milled off as the Clinton gun ban was on the horizon... Interestingly the lowers on these guns are marked A2 but are actually SP1 lowers with non fenced mag release, large front hinge hole with slotted screw head pin .





Besides the fenced mag release the most notable difference between A1 and A2 lowers is that A2 lowers are reinforced just above the rear take down pin , Basically finding a Colt marked A1 lower with the fenced mag release made before the A2 reinforcement is the hardest part, IIRC the only source for those is buying a stripped Nodak Spud lower then having it finished to match your upper and marked by an engraver to replicate the M16.

I was on the fence about building one from an X USGI M16 upper (Cheaper than dirt was selling these several years ago for $350) they now sell for around $550 to $600 but by the time you scrounge up all the other parts you will be in over $1000 for an authentic looking parts gun copy, there are a couple manufacturers now making A1 copies but they are few and far between,
The DPMS lite 20 is a mixture of A1 and A2 parts with the old style rear site but has a brass deflector so is actually a copy of the Canadian C1, (below)


the other is the Olympic Plinker plus 20 which is a mixed bag of A1 and A2 , basically an A1 upper mated to an A2 lower below, ( BTW I havent seen a right side view yet)


IMO $2400 is way too much for a retro M16A1 when new Colt A4 flat tops are around $1000 and a much improved design, perhaps since sales are so sluggish Colt will stop being so greedy and make a run priced in the $1000 range once interest peeters out.