That looks to me more like an "F", not that I'd know what FLB would mean any more than ELB.
This is an interesting beast. I have never seen an anvil with a bolted top, especially if it isn't welded onto the body in addition to the bolts. Likely just my inexperience. Or, it may not be the original top. It is a pity the face was used for cutting and has left those deep scars. Nonetheless, if you don't get answers here, you might try anvilfire or iforgeiron and post the pics and question there.
Good luck! Very intriguing "oddball". Please let us know if you discover its provenance.