Again with Roland's permission, I am copying below, in its entirety, another of his posts to the A-P Users' Group. It adds significantly to the topic of this thread, reinforcing and extending comments in his earlier post, quoted above.
The new post underscores his earlier comments with the observations that FPL53 "has absolutely no optical advantages over the more modern glasses of Ohara FPL55 and Hoya FCD100," adding later that it "is getting to be an obsolete glass."
There could hardly be more definitive and informed answers to the question posed by the OP.
From: "Roland Christen via groups.io" <email@example.com>
Subject: Re: [ap-ug] Glass availability
Date: January 7, 2021 at 7:05:02 PM PST
To: "firstname.lastname@example.org" <email@example.com>
FPL53 is very expensive because it is difficult to make and is subject to de-vitrification if not processed in a very tight range of temperatures. It is quite difficult to polish due to its softness and is easily broken due to its fragility. It is getting to be an obsolete glass and only available in pressings up to 6" diameter and strip up to 5.5 to 5.8" width at enormous cost. It has absolutely no optical advantages over the more modern glasses of Ohara FPL55 and Hoya FCD100. In fact, there exists a slightly better combination of 3 elements to almost perfect color error with the Hoya glass versus the Ohara FPL55.
Russian OK4 glass is a distant second in suitability for color correction because it has a lower Vd value (Vd=92 vs Vd=95 for FPL53, FPL55 and FCD100). OK4 is closer in characteristics to Ohara's obsolete FPL52.
Schott has their own versions in the form of FK54, which is equivalent to the obsolete FPL52 and the newer FK56 which is close to Ohara FPL55 and Hoya FCD100. Personally I would never buy any of the Schott glasses due to some very bad experiences in the past where they shipped very poorly annealed glass with horrendous internal homogeneity and refused to admit to it, even after having been shown the test data.
For me, both Ohara and now Hoya have produced nothing but top quality material and after processing a thousand blanks had only one bad piece in a 175mm blank from Ohara.
Today, the more modern super ED glasses have harder surfaces, are easier to polish to a high level and much less brittle and subject to damage during fabrication. Optically they are truly super glasses. I don't know why anyone in their right mind would make lenses out of FPL53 anymore when there are superior glasses available. Could anyone tell if FPL53 is in a lens versus FPL55 or FCD100 or FK56? No way, the difference cannot really be measured. Any slight difference is totally swamped by the mating elements, because the choice of mate is the thing that determines the final color correction.
This is not the final analysis of available glasses because there are now some lower cost ED glasses made in China that may come close in performance to the Japanese glasses. However, so far they remain in secondary position (not proven quality yet).
As far as availability, we have no problem at this time for the lenses that we want to make. But in order to get the glass made, we do have to commit to a large quantity. Getting single pieces of ED glass is problematic and hugely expensive.