I have not seen a manual for the Plessey 250 but this book has a good chapter on it. That machine is what I would call a segregated capability system. Capabilities can be stored at addresses, but only some addresses and not others. A page in memory is either only for data or only for capabilities. There are a very few universally trusted programs that have some pages mapped both ways at once. Keykos has ‘number keys’ which is an artifice that lets you store 88 bits of information in a place otherwise destined to store a capability. Indeed we insist on calling it a cap (actually a ‘key’) for uniformity. It is exactly like a capability to read an immutable location with those 88 bits.
Architectural Features of Ststem 250 D. M. England. In Infotech State of the Art Report on Operating Systems. Infotech 1972.