Hello Peter,
you are right, my 409 is chassis #7355 and was actually the first to be issued with power steering as a series equipment. I bought it sight unseen via phone also for this peculiarity as it marks the beginning of a unofficial 'MkII' series, never sanctioned by Bristol as such but justified by several details; the change of engine numbering is one of these changes.
You can see more details of my 409-7355 here:
My Favourite Cars-Bristol 409
Best wishes to you all,
Stefano