Here are a few things you guys might check:

Roll mark was changed from Patent Firearms to Firearms Division between 74284 & 76278, can we narrow it down?

Between 162XX and 25XXX the raised plus on the right side of the lower was no longer used.  You that have SP1's in this serial number range, can you check yours?

Colt stopped pinning the receiver extention to the receiver after serial number 087XX and before serial number 10095.  Anyone have a SP1 between these two?  On the SP1's below 08800 is there a flat on the barrel extention for a wrench?

Hope this helps.

Colt SP1 AR15, semi-auto, 20"bbl, year of build 1973 and S/N 208xx
Bbl has C MP C on bottom about 2" back from FH and 6 slot birdcage FH.
Bolt carrier assy has a C stamp on left side and bolt has no markings.
There are no markings on upper reciever. FSB has a C on right upper side and a key hole (bell shape ?) on right side of bar. Has a rivet on front slang loop.
Lower has RAMPEP PONY in circle next to COLT AR-15, next line CAL..223, next line MODEL SP1, next line SER.SP208xx. Next behind bolt release. COLT'S PATENT FIREARMS MFG.CO next line
HARTFORD, CONN. next line U.S.A.
Rifle was bought NIB late summer 1973.
Saw a ? about 1973.
Hope this helps.

ETA: hammer has a C on left top side, disconnect has no marks and trigger has a number 2 on left lower side, no other marks found. Rifle came with Colt 20rd  mag with removeable 10rd block.

SP24431, 1973 manufacture.

1978

SN 921xx

Carbine.  Unmarked FSB, Barrel was marked C MP CHROME BORE, but has been cut to 11. 5 inches, as the piece was Class 3d and made into an XM177E2 clone in 1979.  Can't give you original upper forge markings.  Upper was replaced w/a big hole forward assist type some years back, and idiot me sold the smooth side upper off.

5 ARs later, it's still my favorite

Updated, and SPII data base here:

ar15.com/forums/topic.html?b=3&f=118&t=272522