is there a way to tell which upper a SP1 had on it when it left the factory?

I'd like to know if the original upper on my SP1 was a 20in a1 or a 16in a1.

Maybe Colt could tell you by the serial number?

Quoted: Maybe Colt could tell you by the serial number?

i could.
yet i was hoping someone had a reference link.

Colt didn't split out SP1 serial numbers based on whether it was a rifle or carbine.

The only way to be sure is to call Colt.  
Usually they will tell you a production date, but I'm not sure if they'll tell you whether it was a rifle or carbine.

www.colt.com/CMCI/archive_prices.asp

2003 COLT ARCHIVE SERVICES PRICE LIST

RIFLES  
AR-15 Sporter, $100.00  
*See Premium Pricing for additional charges*


PHONE SERVICE  
This premium service enables our customers to obtain all the pertinent information on first generation Single Actions up to the 343,000 serial number range over the phone. This does not include the Bisley Model. A historical document containing the information will follow within two to three weeks. A Mastercard, Visa, American Express or Discovercard is required as a method of payment for this service. This expedited service is also offered on the 2nd generation Single Actions. However, we require one day to process these requests

Dates of Manufacture
$25.00  

Identification Service
This service provides novice collectors/customers with identification and general information on a specific firearm
$50.00  

for that kind of money i will just guess.