My M4 Pro Macbook Pro I web-ordered with storage and power upgrades from the U.S. Apple online store in March has an "L" serial number.
Long ago, Apple used the first two letters of the serial number to show the factory where it was assembled. As far as I can tell, that pattern is long obsolete.
The only place where the first letter appears to have any significance is in iPads, iPhones, and later Apple TVs. There is a "order number" in this rough format — XX000XX/X where "X" represents letter and "0" represents digits—that shows options and localizations. The first letter options is M, F, P, or N.
— If the order number starts with M, it was purchased new from Apple or a major carrier.
— If the order number starts with F, it was refurbished by Apple or a carrier.
— If the order number starts with P, it was sold as a personalized device with Apple engraving.
— If the order number starts with N, Apple provided it as a replacement device for a device returned for service
I sent my older iPad in for a battery service about four years ago and, as usual, they quickly sent a refurbed replacement of exactly the same specs with a new battery and new outer shell. It's order number:

That shows after tapping the model number in Settings on iPhones and iPads.