That’s an OEM part number. The actual part is probably NSL5910 from
Luna Advanced Photonics (formerly Silonex), but there’s almost certainly a selection process at UA not only to match the cells for a given T4b, but also to select for the general photoresistor characteristics. Modern parts are inherently faster than the original Clairex CL505L that the 5910 replaces, so testing/sorting the LDRs is needed if you’re going for vintage characteristics. On the datasheets, CL505L and NSL5910 are identical. But in reality the batch bell curves for light and dark response times are not. UA and companies like Kenetek sweat those details. Some DIYers do and some don’t.
Given that only one of your photocells is dead, you’re likely to have to buy a fairly good sized batch to find a match for the good one. The cheaper alternative might be to find a new set of matched cells from a batch and replace them both. I say “might” because there’s a good chance that the unit will have different characteristics than what you originally had, which may or may not be noticeable to your ears. If that’s not acceptable to you, then you’ll just have to buy enough cells to find a match for the good one you have.
To see differences in a new set of cells vs your original good cell, you can do a compressed-signal amplitude vs time plot with the one good cell in the gain reduction position (as opposed to the meter position). Then do another identical test (using the same source material) with the new cells installed. Looking at the resulting pictures one on top of the other will show you any differences in attack/release times and in gain reduction level.
David Kulka from Studio Electronics has T4 stuff here and there. His ties to UA make him the first source to try IMO.
I’ve also had good luck with the cells that Frank at Curious Audio Parts sells. I have a small stash of original Clairex cells to compare to and the ones he sells are as close as you’ll find, as measured with the basic tests I’ve used. No idea how he sourced them. Really nice guy, too.
https://sites.google.com/site/diypartsstore/catalog/new-t4b-opto-el-panels
As far as testing a T4 or LDRs goes, there are many posts covering aspects of that subject over the years. Do a search on “T4b testing”, “photocell testing”, “photoresistor testing” etc and you’ll find quite a few methods for testing various aspects (and a lot of debate).
