Engine drive belt pulleys

I spend my evenings poring over printouts of the technical diagrams that I am accumulating from various websites.  Trouble is, this then leaves my mind tossing and turning over unanswered questions, and so it was up with the lark to get an hour of engine investigation in before work.  I am still confused about this elusive 'Beltomatic' that I read about here and there.

I have been trying to get the flywheel off the crankshaft, but it really doesn't want to budge so I am leaving it to soak in penetrating oil, and turned my attention to the belt drive side of the engine.
Concerned about possible left-hand threads (I miss a Haynes Manual) I calculated from the rotation of the rear wheel back through the drive-train that the retaining nuts and bolts here should all be standard right-hand threads - I was right, and it wasn't long before I had the adjustable pulleys off and on the bench.

So what did I find?
Well I appear to have the 'triangular' lobed drive shafts that are identified in the tech drawing as a 'possibility' - does this provide any insight into the rarity of the engine? or the place / date of manufacture?
I was amazed to find that main bearing oil seal looks like new!  no oil or crud (the photo shows exactly as I found it) unlike the transmission box oil seal that looks a bit gunky

transmission shaft is of the triangular lobe type

Beautifully clean main bearing oil seal in the background

Backside of drive pulley - flyweights all sound

Outboard design of drive pulley gives clue to the engine age and model?

Note the triangular lobed driven shaft 'option'

So an interesting hour.  I think I am understanding this a bit more. BUT i still cant fathom that peculiar assembly in the bottom right hand corner of the diagram (this is all missing from my engine) What does it do? (I think its a safety 'dissengagement' device - eg a sort of neutral for when starting the engine) But how does it work?