.. so it must have been an upgrade kit?
To fit a Plus logic board into a 512k bucket, you only have to remove the tiny pillar between the two D9 serial ports. The SCSI ports sits there, and one of the Plus's DIN-9 serial ports is accesible neatly through the microphone-labelled hole on the right. The second serial port remains covered. For the moment, this is how I share my 512 with a Plus board until I can find a stock bucket for it.
Rick