Creaky ,i posted a reply but it appears to have gone AWOL.I've looked at my spares,the cradle has a slot like 5/8" long (primary end of cradle)
but as you say there's not real slots on the timing side of the motor,although the manual says the later Magdyno is adjustable for slack.
I have a spare dyno distributor CS35 off a Meteor and it has the duplex sprocket,also the manuals exploded view of the motor confirms that.
Check it has the stud-nut on the cradle looking in from the primary side.If not you may have a 500

.The single chain would suggest
a Twin 500.together with the 26mm carb fitted beforehand,Meteor eng no is T7.... Chain should have 3/16" up and down slack.Also there
should be no play in the dizzie shaft,I have 53 and 54 rotors,like Mark says toss them for new ones.
cheers
colin