There are two different types of aluminum Bushmaster lower receivers. Some have an "L" prefix to the serial number and some have a "BFI". There were also some limited editions with different prefixes but 99% of them will have one of these two prefixes. So the serial number will either say "L123456" or "BFI123456" This is never any space between the letters and the serial numbers.
There will be the snake logo, then under this the serial number. Under the serial it will say "CAL .223-5.56MM" or "450" or "Multi-Cal". Under this it will say "MOD.XM15-E2S". Also the snake logo itself is a slightly different design on the "BFI" marked lowers. The "L" lowers use an older design of snake logo.
Then over to the right of the bolt catch it will say:
We've seen counterfeits from different countries before which actually look very close to our logo. The one I have seen has a bigger serial number with the wrong prefix and the snake logo looks a little different when compared side by side. People have also altered serial numbers to make their own "pre ban" lowers. If you ever have a question about a certain lower, contact us at 1-800-883-6229 with the serial number and we can usually provide you with the information of when it was manufactured and where it was originally shipped.