Around 2011, Colt transitioned from having a seperate ‘Sporter’ product line and ‘Law Enforcement’ product line to just a single product line that retained the LE serial number prefix and used the M4 Carbine marking on the lower receiver.
The Sporter product line had an SP serial number prefix and ‘Sporter’ on the lower receiver.
The Law Enforcement product line had an LE serial number prefix and Law Enforcement on the lower receiver.
During the transition from the Sporter marked lowers and the use of the ‘M4 Carbine’ marked lowers a small number of SP serial numbers were used on ‘M4 Carbine’ marked lowers before the SP serial number series was stopped completely.
I recently aquired one of these transitional carbines. It is an LE6920MP-B…SP serial number but M4 Carbine marked lower. I don’t know that I would really classify this as anything rare necessarily. I find it interesting more than anything as it is a product snapshot during one of Colts many product transitional times.
MP-B in the model number represents ‘MagPul – Black’.