I recently purchased a PB Touch HD 3 and noticed that it came with Software version: U632.5.19.1055, Release date: 20191219_020134 ...

I see that https://www.pocketbook-int.com/int/support/pocketbook-touch-hd-3 only has versions 5.19.832, 6.0.926, and 6.1.900 available for download.

Does anyone know where I could get 5.19.1055, in case I wanted to reload my original firmware on my device after trying out the newer versions?
Can't find it. Maybe this release had problems and it was withdrawn? Which might make sense since they have an older (5.19.x) "fall back" version available for download. But you can email PocketBook support. They responded within hours the one time I needed to get hold of them.
I'm now in the same position as you. I received my HD3 today, it came with 5.19.1055 — I'm tempted to upgrade but the firmware on it seems to work really well. What if the upgrade doesn't?

I think I'll write to PocketBook and see if they can make 5.19.1055 available for personal download.
I ended up upgrading to 6.1.900 - it seems just as fast as 5.19.1055 to me so far, and I like the interface better!
Okay, that sounds good. I've sent an email to PocketBook to see if they can provide a 5.19.1055 download link. If not, I'm not sure yet what I'll do — though, reading the 6.1.900 release notes it looks like they made a LOT of fixes/improvements in that firmware release.

Thanks for the update.