Guns weren't made strictly on engineering change mandates. They were made from parts that had been made and put into bins to be used as needed, and/or until used up. Your gun would be an early -2, so the L on the cylinder is not surprising.

Frames were not used in order of number, but were simply grabbed from a parts bin and used as needed. The OP's could have easily been serialized, then grabbed out of the parts bin, stamped 28-1 and used to build the gun he has before the mandate for the dash two engineering change came down. I'm not sure how that would work exactly with the change to eliminate the hole in front of the trigger guard for the stop screw. Perhaps some 4 screw frames were used up for the very early -2 guns?