HP-92 query

[HP-12C]

Hi

I am fairly new to the HP-92 and have a few questions please.

Was there only one HP-92 model? Are all units the same, or are there different revisions or models within the HP-92 model?

I notice there isn't really any information out there on these other than the HP Museum here, there is only one video on YouTube, are these units quite rare? They do seem quite expensive when they do appear for sale.

Thanks for any help