Noticed this Altair on ebay has a hex display on the front panel, was that kit that existed or is this someone's diy? It's very neatly done.
https://www.ebay.co.uk/itm/235023012435
A couple of memory boards and a mystery CPU board, they also have more of the mystery board for sale in another listing
https://www.ebay.co.uk/itm/266243970013
It appears that the mystery board has been discussed previously...