I buyed it.
The front looks like a modified DP501 and the holes for LEDs are drilled afterwards. The front is also not a typical laminate for the late series, its a thick plastic ( ~1 mm ) . I never had seen such a front type.
The other Rubidium Standard plugin with the blue PCBs are selfmade. I owned these PCBs also . Version 1 PCB is not really ok, wrong holes and wrong contact spacing.