XM-15E2S is a generic marking for the Bushmaster lowers. I built about 150 rifles on XM-15E2S marked lowers when they were still widely available. My 16” Hbar A1 M4gery, a factory model, has the XM-15E2S marking, as does my Varmint Special Stainless 24” factory rifle.
The 20” Hbar and the 16” carbine were most commonly called the E2S models, but the stamping was rather generic across their AR-15’s.
My Hbar has been my most reliable AR for consuming rot-gut ammo I have ever handled, out of several hundreds of AR’s. If a round can be made to go bang, it’ll go bang in my Hbar. I’ve shot hundreds of coyotes with it, and have done all of my instructor certification requalifications and my class demonstration shooting with it.
The 20” Hbar E2S is a great rifle in its class. For the right money, if a 20” using rifle is your objective, it’s the right rifle.