This was a post I made a while back that may help on vent styles. Also some pictures of my NOS recirculating heater that very clearly shows 2-different sizes of hose and no bushings. My take is it is a mixed bag.
Two styles of 1954 vents both made of aluminium one type straight the other reverse bend see pictures:
http://good-times.webshots.com/photo/2586236450102454676qRPcYh
http://good-times.webshots.com/photo/2021907660102454676cpkuJb
Next there are at least two styles of sheet ducts: 47-51 style:
http://good-times.webshots.com/photo/2405252410102454676ppisPX

52-53 1/2 ducts:
http://good-times.webshots.com/photo/2809720060102454676gVgVNT
http://good-times.webshots.com/photo/2653102730102454676PXUNwF
Then to add even more confusion I have seen them with and without these black spacers (usually on 1947-1950 trucks).
http://good-times.webshots.com/photo/2661175600102454676CudWvK
http://good-times.webshots.com/photo/2246331650102454676MRFRlr
The driver side kicking right a bit depending on the style.
Both factory heaters with hoses 1951 picture.
http://good-times.webshots.com/photo/2771261570102454676PZGtET
1952 NOS heater hoses for a recirculating heater no black bushings in kit or in directions.
http://good-times.webshots.com/photo/2411325270102454676ASUkYJ
NOS 1952 diverter and vents w/bushings for comparison you either have to have correct hose diameters or use the bushings and the larger hose.


1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr