The pictures to the right show the clock's interior.
The green PCB with the Nixies mounted on it is a vintage Burroughs plug-in module, with the front corners rouned off to fit within the clear plastic dome. I have no idea what sort of equipment it was intended for - I've never heard of an original Nixie device with a 2-digit display (certainly nothing at the time would have used a sequential display like this clock), and the card is much too wide to conveniently use in a stacked wider display.
I was unable to find a matching edge connector (the card uses an oddball 0.15" spacing, rather than the common 0.156"), so the card is simply soldered to the main board. Tube replacement is intended to be done via desoldering the tubes themselves, not replacing the entire card.
The clock is a bit top-heavy, since I had to mount the transformer above the Nixie card. I'm just happy to have found a Nixie-suitable power transformer, a NOS Stancor P-8624 with 125V and 12.6V secondaries.