Can anyone out there throw any light on the origins and reasons for the "Robin" manifolds and carburettors that were often fitted (after market product) to Wallis 20-30 tractors?

They are most comonly found on 20-30s of Candian origin like mine - see the picture.
Also they seem to be exclusively found as gas (petrol!) only tractors.

I have had several reasons proposed to me:
1. More robust and to replace original poorer quality Wallis manifolds.  Certianly the Robin manifolds are of far better quality steel
2. Gas cheaper in Canada at the time
3. Better performance in cold Candian conditions
4. Increase in power 

I am currently renovating an original Wallis manifold/carburettor assembly to bring the tractor back to original spec.  Needless to say it's an arm and a leg job and took me 10 years to find a set that was suitable for repair and restoration.

All as I can say is that the little Robin carburettor is very reliable.  I have a styled 25 from Canada but this has retained the original Kingston carburettor.  This too I am reconverting to original after another long search for a semi decent manifold set.

John