I don't know what "F" stands for, but

Y = "Notably higher response level in the region 2 to 6 kHz"
X = "Same as Y, but a wider frequency range"

In radio's with extended short wave bands you often find "X" type speakers, i think because these radio's where often used to listen to speech.

I studied a few service documentations and found for the BX type radio's from the early 50's to about '52-'53 they have speakers that where "repairable". Sometimes only parts where available, not complete speakers. In that case the speaker seems to be a part that was specially made for internal production for radio's only, which COULD explain why there is only a codenumber on your speakers.
From about '52 -'53 the sevice docs mention "not repairable speaker" which means the complete speaker must have been available at least for service purpose.

Maarten Bakker, also a member of this forum is known to know VERY much about Philips codenumbers. Maybe he can spread some more light about this numbers?