BMC to Newage Coventry Coupling Plate?

I have been advised that one or two of you technical wizzes may be able
to help me.
I recently had my second-hand BMC 2.52 narrowboat engine rebuilt by
Peter Atkinson at Farncombe Boathouse on the River Wey. He was THE man
for marine BMCs. Unfortunately he died two weeks ago.
Luckily for me he had rebuilt the engine and bench tested it before
that sad day, so things could have been far worse.
However, the marine gearbox, a Newage Coventry Ser No: 24-14565 is
still off the engine. I can't find a type plate on the box, but it is
one of the bigger ones about 25 years old [though hardly used, like the
engine.]
There is a circular aluminium drive plate that bolts on to the engine
flywheel. It has three shaped lugs on it facing back towards the
gearbox. On the gearbox shaft is a similar circular plate made of
steel, also with shaped lugs projecting forward. It appears to be held
in place by a bolt tapped into the end of the splined shaft. The
'shapes' on the lugs don't match up, but the two halves face up
together nicely, giving about a quarter of an inch play in rotation.
When Peter stripped the engine he told me that 'rubber biscuits'
should have been fitted between the lugs, but they were never there!
However, as the engine had only done about 30-40 hours running in its
25 plus years, no real wear had taken place on the lugs.
Having inspected the drive coupling myself, I don't see how loose
rubber 'biscuits' could be located on assembly. After all, the coupling
is inside the bellhousing, so on assembly you couldn't get at it. But
also if the coupling is bolted on to the gearbox shaft, you couldn't
get to the flywheel bolts either!
I am now wondering if these two plates, one on the engine, and one on
the gearbox shaft are in fact two halves of a coupling assembly that
broke apart when Peter took the gearbox off. The only other gearbox
drive couplings I have seen were shock-absorbing assemblies that
replaced a clutch.
I suspect I will have to find a new coupling from somewhere.
Can any of you stationary engine types shed any light?
Much appreciated.
Tony Haynes
nb Dreamcatcher
Basinsgtoke Canal Centre