The Commodore PET series had empty sockets reserved for upgrades. Both official and third parties employed this method as well as removing the CPU and hijacking its signals... However it is very nice to see a card upgrade like the Apple ][. Apparently it contains a backplane for DIN-41612-64AC sockets...

By the way, there were various iterations of the 2001. Yours is not the first one, but a later one. The memories employed in the first PET-2001 were made by MOS and were wider than the ones you have, which I presume they are regular 2114. They have the same capacity but differ in the amount of select signals.

They would later be followed by the second-generation PETs, labelled as CBM 30xx series in Europe (still labelled as 2001 in USA). Those would employ dynamic memory as work RAM, have a non-chiclet keyboard and remove the integrated cassette.

Finally they would be replaced by the final generation of PET, which would remove a lot of 7400 ICs from the video subsection and base it around the 6545 (similar, almost 100% compatible with 6845 although there are some differences). There would be models that retained the 40 columns of text and others, upgraded with more video memory, with 80 columns. That would be the 40xx/8xxx/9000 series.

I heavily advise you against reverting the modifications in your unit. They were contemorary and the whole community could learn from them. In other words it gives your unit more personality. Can't advise about the economic worth, as it is difficult to grade (especially with those mods).