I suspect that the webmaster at GoMoG does his best to provide as much information on Morgans as may be humanly possible in the hope of helping those trying to keep their Morgans on the road, but if the MMC can not supply a handbook with an accurate wiring diagram for a Morgan at the time it was new, I suspect some time down the road folk are bound to be scratching their head trying to resolve wiring issues...? (-:

I suspect that around the time your Morgan was built it would incorporate standard Lucas components which can be found in use in many other UK manufactured vehicles of the same era, thus with a bit of searching you may find circuit layouts for those components on MG or Land Rover Forums and the like...?

http://www.advanceautowire.com/mgb.pdf

As for fuse ratings that is perhaps a tad more complex than imagined to work out what any fuse will blow at and involves a bit of mathematics to fully understand. I bow to Bloggins knowledge of fusing factor variations relative to differences between the UK and US, for I have no such knowledge, though do have a measure of understanding of fuses with a dual rating in industrial circuits and which carry an identification as being "M" rated for electric motors, given a motor can take circa seven times the normal running current at start up, before an electric motor gets up to speed and the current drops to normal... Wire fuses are pretty coarse means of protecting wiring and generally identified relative to the normal current draw expected in the circuit, but it can take many more amps than the stated rating for the fuse to actually blow., it can all get rather technical requiring both time and current to be factored into an equation....Yeah it can all get a bit scientific, when all that is required is to select the correct fuse for a circuit

https://en.wikipedia.org/wiki/Fuse_(automotive)

Perhaps best to be sure you understand the likely current to be found in a circuit before attaching an ammeter across the fuse holder for you could easily blow a meter circuit if it is not capable of carrying the current likely to be found at that location..

There are those better qualified than I who may be of far more assistance to you but happy to ramble on a bit if you think it might help.

Good luck.