Velvet Drive Mystery

I have installed a left hand 4108 Perkins with velvet drive in a Hooper Island Draketail. The velvet drive has no ID plate, the series number intergral to the port side of the drive housing reads WG DIY 72 1-F, there is a circled "B"next to the series number. The only plate on the velvet drive housing is small and way down under a circular boss port side facing forward - it reads "22 0".
My question: how do I determine the gear ratio?
I have been told it is either a 1:5 or a 2:1 drive. I am fixing to put a 14 / 4 prop one her just to see what happens.................

 

Velvet Drive transmissions are 1:1. They can have a reduction gear bolted to the rear end. Does yours have it?

 

I ended up doing a photo tach, it was 2:1

 

can you add the reduction gear to any 1:1 Velvet transmission?