Although I think that whoever answered your question may not have known exactly what you were looking for if you had done a little more research you would've been able to find what you need."I'm building a semi auto M16-A2 clone, and I'm having a hard time finding a non-"F" marked front sight base. I'd even be happy to buy a used one that's been taken off a rifle, and I'm hoping you guys might have something like that."
Here's their answer:
"All parts we carry are made to milspec. Milspec front sight bases which would be required for a M16 build are all “F marked”, which signifies they are forged milspec front sight bases."