The mystery R733 coaches

Picked up on a second-hand stall for four quid each, these Tri-ang/Hornby coaches are a bit of a mystery. 

Described as "Swedish" coaches, they we supplied in the RS2 clockwork locomotive pack between 1969 and 71. The 1970 catalogue has a very impressive photo of the contents of the set. 

As far as I can tell, the have the catalogue number R733. That's from several sources - but if you search for this, the results are often the 6-wheel Palethorpes sausage van. Was the number re-used? Were the coaches available seperately? 

I'm assuming that the "Swedish" name attached to the model relates to a prototype, but I can't find a relevant photo, nor am I an expert on that countries rolling stock. There are some Austrian coaches, also found on the Welshpool railway that look very similar though.

These are, to my eyes, attractive models and I've seen them for sale in the past. There used to be piles of the plastic bodies too. I know there is one in a cupboard somewhere I repainted. Why were there so many spare bodies, and where have they all gone? 

The chassis is a bit odd. It looks a lot like a Pacer, long before the bus/train hybrids were ever thought of. 

My pair have slightly different chassis. One brown and one black. The black one is missing the centre "door" on the ends. Why tool for two variants on a freelance coach? 

Repainted, and possibly with a new chassis, this would make a nice set on a freelance branchline model, pulled by a small loco in a Colonel Stephens style. 

Finally, I have a found a video review of the train set.