The first batch of 25 resonators:
The second batch from the same eBay source arrived a few months later and I finally got around to measuring them:
A dot of green Sharpie on the AT26 cans identifies the second batch:
The alert reader will notice an un-measured 25th resonator at the bottom of the first batch. I dropped one from the second batch under the Electronics Workbench, found it, then also found its long-missing brother; now I have a genuine it’s-never-been-used resonator, just in case the need arises.
A quick-and-dirty simulation shows the series and parallel resonant peaks come out close, but not dead on, the actual measurements:
The model obviously doesn’t exactly match reality, which isn’t too surprising. However, I don’t understand something about tuning fork resonators, because the parallel resonance shouldn’t shift upward with the series resonant peak when the circuit gains a 24 pF series capacitance:
Suffice it to say that doesn’t happen with the simulation.
More study is needed, as the saying goes.
The Smell of Molten Projects in the Morning 
I’m curious about that shifting parallel resonance too. I suspect your implication is correct, and it’s (somehow) an effect of the tuning fork structure (versus the usual twist/shear crystals). It would be tempting to measure a conventional crystal of a similar frequency and see if the same effect appears. At first I figured such things would be unwieldy and nonexistent, but I remembered I have a 100kHz crystal, a pretty little thing with gold contacts housed in a glass housing the same as a 9-pin miniature vacuum tube, and probably a couple in oversized HC-6 packages. So, do I duplicate your device, or send you a couple for testing?
This is America: we can do both!
The least horrible alternative seems to be sending a couple here. Pack ’em well, I’ll take good care of ’em, and return ’em promptly. Thanks!
Near parallel resonance, the resonator is mostly inductive and one could model it as a single RL (without internal capacitors). Then the RL would resonate with CX in series mode, which might just account for what I see.
On the other paw, I can’t find any resonator measurements at 0.1 Hz frequency resolution, so the simple circuit model probably doesn’t capture the physical reality under these absurd conditions. I vaguely recall a more complex model festooned with inductors & capacitors, but …