...which is darn near as useful as a buggywhip, these days.
Got two rails of PCA 1829 10Base-T transformers in DIPs in a mixed lot of NOS crap which had some more obviously useful things in it, as well as some that are probably less useful, and just a few bits that look not actually NOS.
Closest I can find to an applicable datasheet, though mine lack the "B" at the end:
Turns out they make a dandy Joule Thief (blocking oscillator)
(vampire flashlight) transformer - two, in fact (I suppose next I see if cutting the package in half destroys them or not - it's 12 pins for two center-tapped transformers in one 16 pin sized package with no pins in the middle, so it seems plausible that it's two separated cores in there.) Considering the low likelihood that I'll be building any10Base-T interfaces from scratch, that's a much better outcome than "hmm, obsolete, useless." I suppose to some extent they would be a small pulse/isolation transformer in a general sense, though I have not investigated that further as yet.