If anyone finds a reference to its existence we would appreciate the details please. Bauer and DeRemer's Compiler Construction text speculates about ALGOL 68 on a B5500 but we've not found any evidence so far it was attempted. As you will see about the discussion of ALGOL-68R (https://en.wikipedia.org/wiki/ALGOL_68-R - the first implementation of ALGOL 68) it took considerable resources to support it, likely more than the B5500 could provision.
