Following is some info on these from one of the internet forums Forum on Vintage amps etc.:
Re: JBL D123 vs. the more famous D120 by Johnny Savant on Wed Apr 09, 2003 5:00 am
The D120f was the model designed especially for musical instrument amplifiers. It has a more rugged frame to preven twisting when the nuts are tightened to much. It also has a slightly wider voice could gap. Apparently people started using hi-fi and sound reinforcement JBLS in guitar cabs and were blowing them up easily. So, an engineer at JBL , Harvey Gerst, redesigned them to take more abuse. The "f" designation was because Fender was their largest customer at the time. But other amp manufacturers used them, too. The D123 was a hi-hi/sound reinforcement speaker that was often mounted in a wall, hence the shallow design. Not a great guitar speaker. Gerst used the magnet from the D123 and the basket from the LE-10 to make the D110f.

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