It seems my DP-FE is a type 2 or "early" type 3 which is why the adapter doesn't fit. Subal have proposed a not inexpensive solution which is that we replace the plate behind the dome to a new type 4 plate. Much cheaper than buying a new or secondhand DP. All my ports will then be type 4 with only my DP-100/3 requiring the adapter.
Having already gone off topic, I'll say this about the ND500... the button labels are tiny! Do you know anyone who makes bigger housing label decals?