I'm attempting here a summary of what I've learned so far from what has turned out to be a very instructive exercise.
1. Mating type for FPL53:
Although 'ZKN7' (Schott N-ZK7) is often cited as the 'ideal' mating type for FPL53, if I've gotten the various values correct in the posted table, N-BK7 looks to be even better in key respects. Compared to N-ZK7 as a match for FPL53, N-BK7 appears to have a more favorable PF,e value, a more favorable Pg,F value, and a somewhat higher refractive index, although a somewhat higher Vd value.
Further, contrary to the suggestion by Thomas Back in his 'Brief History of Astro-Physics Lenses' that Astro-Physics used ZNK7 with FPL53, Roland has frequently noted practical problems with use of ZNK7 causing him to abandon its use, and Alan French has put it simply in various places of one thread "AP has never made any telescopes using FPL53 with ZKN7." "The AP lenses with FPL53 did not use ZKN7. Because of issues with glass quality, AP had stopped using ZKN7 even before the adoption of FPL53." and " If Tom Back claimed AP used ZKN7 with FPL53, he was mistaken."
BK7 is the only mating type I've seen specified for an A-P scope -- used with FPL53 in the relatively recent 175 f8 EDF triplet, with the best specified color correction among A-P scopes up to its time. Sky-Watcher also uses the same FPL53/BK7 combination for its admired ProED doublets (80mm, 100mm, 120mm).
2. FPL55 vs FPL53:
Again, from the values in my table, if BK7 is chosen as the mating type, FPL55 looks to be at least as good, maybe even better, a match than FPL53; FPL55 has a value of PF,e even closer to BK7's, the same value of Pg,F as FPL53, a somewhat higher refractive index than FPL53, and even a slightly higher Vd.
Vla's comparisons in this thread of FPL53/KZFS2 and FPL55/KZFS2 doublets showed those two FPL glasses to provide very close results, very slightly better ones with FPL53 (p.3 of this thread, post #65). Although it's not clear that this is the explanation for the slightly better performance of FPL53 in these particular comparisons, the table shows a PF,e value for FPL53 very slightly closer to that of KZFS2 than is the PF,e of FPL55. The reverse is the case for BK7, favoring FPL55 as just noted.
It would be interesting to see models of doublets and triplets comparing results with the FPL55/BK7 combination to results with FPL53/BK7.
In any case, the model comparisons Vla has already contributed to this thread do suggest that, in theoretical terms, there's 'not much' wrong with FPL55 compared to FPL53.
3. FPL53 vs CaF2:
Alan French (bravely) initiated a recent CN thread discussing this comparison, with reference to the examples Vla provided in his telescope-optics.net pages:
The telescope-optics.net page examples cited by Alan compared doublets (100 f10 FPL53 mated with ZNK7 vs CaF2 mated with K5, example #15 vs #16) and triplets (140 f7 FPL53 mated with ZNK7 vs. CaF2 mated with K7, examples #18 vs. #19). To these can be added the doublet comparisons, in both Fraunhofer and Steinheil configurations, (100 f7 FPL53 mated with ZNK7 vs. CaF2 mated with K5, examples #12 vs. #14).
In all cases, The FPL53/ZKN7 versions yielded polychromatic Strehl values as good as, or slightly better than, those from the CaF2 combinations.
Vla's models in this thread of the FCD100/ZNK7 combination in doublet and triplet designs and of FCD100-based triplets using combinations of differing mating types chosen to 'offset each other's deviations' from ideal matches -- H-K5/FCD100/N-BK7 and H-ZK1/FCD100/H-K9L -- point to the latter path as the most likely preferable one to be taken with FCD100. Vla noted that something like the second version comes close to what Roland has cited as the degree of correction in the new Stowaway 92 (p2 of this thread, post #30 and post #48).
It is interesting that Hoya's introduction of FCD100, April 20, 2011, included the comment "Combining it with optical glass of high refractive index allows us to achieve unprecedented advanced chromatic aberration correction."
The comment uses the same strong wording of high promise that Roland employed in announcing the new FCD100-based Stowaway 92 -- "unprecedented color correction."
Could FCD100, used with some appropriate mating type, or combination of mating types in a triplet, in fact, be better than either FPL53 or FPL55, with 'unprecedented' correction at least over the main visible range?
Comments pointing out errors in these conclusions and, particularly, ones noting any misunderstandings underlying errors, would be welcome and appreciated.