For most of their service life, redhens were in service, but there was a continual servicing and overhaul regime. So at any time a couple of sets would be out of service on any given day. ( plus failures etc)
The notes that come with the SAR Model Company redhens have a partial listing of differences in various cars. The cars you question were the Rolls Royce motors ones I think and because of that tended to be the first retired. They were noticeably different as they had air tanks outside the bogies at the B end of the cars. It meant the spares could be rationalised.
437 was never built and 300 was not renumbered for computer purposes like many AN locos etc were renumbered, eg 930 to 967 etc.
The rolls Royce and GM units had slightly different power and acceleration characteristics and I was told once that they mixed a GM with a RR to get a bit of extra grunt and also better acceleration. I have never seen an official record, but respected drivers have passed on stories.
It is also the reason why a fourth car was added to the superchooks and was often a RR version.
If you look at the comrails site you will find some images of both series.
The ARHS article by Kim Bird circa 1985 showed several shots of mixed sets. The only up top difference was a variation in the radiators, otherwise the bodies were all built on the same jig. ( 300 and 400 were the same length, shape, height etc.) A couple of very early 300,s were altered as the side sills hit the platform at Ovingham on test trips. The rounded ends on top were simply to fit headlights and to stop drivers trying to drive from the wrong end LOLOLOL!!!!
Internally there were small differences in seat colours, interior pane colours and a few seating changes over the years. Each seat frame was stamped with car number and seat location eg 300 -1 etc.
Towards the end they got mixed around to keep them running.
In the beginning 300 would be paired with 301 etc. But photos exist of a nine car Gawler race train with no order to the numbers. This was in the days pre 1964 when the numbers were set higher up the cars and prior to the gold "speed" stripes.
In later days prior to withdrawals commencing it was common for a set to be arranged with paired numbers.
Also after the withdrawal of the trailer cars it became common for 300 to run with 400 for the provision of a guards area and a first aid station.