By the 1990s, customers were expecting to add many more screens and wanted new functionality from the O/S.
New features were added:
Bank Switching - the bottom 32K was fixed, but multiple different top 32K stores could be switched in and out under O/S control.(Many new customers were running the MCMB package which required 4K partitions. i.e. Half the number of partitions / terminals that might have been expected in a given machine.)Real Time ClockSmarter IO boards that buffered more input from users and hence reduced the load on the interrupt handler.(The BCL designed four port serial that apparently required 4 x 6502 processors!)New OS Commands to 'Stow' a partition to disk and pick it up later at either the same screen or another.New Disk controller that supported (SCSI ?) disks and tapes.And, finally, the front panel with all switches and lights has gone. Replaced with a 'single chip' microprocessor that sat between the M18 and the primary console. I believe a control key sequence was used to switch between this microprocessor and the M18.
I don't have any photopgraphs of a late model Distributor EP. The machine alongside was originally a straight-forward Mk V, but converted on site to an EP. The front panel has obviously gone, replaced with a small LED two line display and a few buttons.