I have a ceramic one without the heatsink that says

IBM9314 Q07001
50H8302 PQ

At least I think that's what it says. They put a slash through the zero's so my eyes can't tell an 8 from a 0. It's hard to read black ink on a black background. There was also a quality control sticker over it and there's still some residue from when I peeled it off.

The number 3 is quite clear, which is odd since it can't be 1993 because that would predate the Cyrix 5x86 by 2 years. Maybe IBM numbered theirs differently from Cyrix? The only other numbers that could easily be confused with a 3 are 0 and 8 and it can't be 1990 for certain and by 1998 these would have been pushed off the foundry lines to make room for newer chips.

Not neccesarily true. SGS Thompson were still making the Cx486DX2-66 in the late 2000's albeit in a different package with pretty much the whole chip-set and GPU. They had a 133MHz chip I have never used which is often documented as "Pentium-Class" and may have been a Cx5x86 core.

These were, however, a BGA plastic package. The chips were employed in SBC's and industrial systems mostly... Though I know 486-era chips lived for some time in SBC's and industrial machinery in their PGA form too, so they may have made them longer for that market.

I thought I remembered seeing some kind of manufacturing date guide for the various IBM/Cyrix processors somewhere. Can't find it though...

feipoa explained it to me, and I will just add that info here.
The string right after 'IBM9314':
P = 1995
Q = 1996
07 = 7th week.