Hello Maarten and all,
Axle, wheels and timelines are a pet subject of mine. I have an extensive record of where wheels and timeline fit into the Lesney era.
It is my belief that the 9.5x20 tread count bpw were not introduced until 1962 and long after a very, very short run of bpw in 1961 when this casting was in production.
The axle length on this model can vary over the life of this brace casting and as a rule axle lengths are between 21.88mm to 22.77mm for models with the roundish brace. Could you measure the three axles on yours please Maarten?
You may find all three axles are different lengths using a vernier calliper.
Thanks for showing this and allowing discussion and in particular taking the time to provide great quality photos.
My thoughts on the genuine 73a bpw version is that there are so few of these genuine examples in existence that the 10x24 bpw were used to finish the last of the round brace casting model. This may have only happened for a matter of hours on the assembly line. Then the last variation with the dies were altered for infill between pump box and tank were ready and production resumed using 9.5x20gpw again.
There is no known recording of a full infill code having bpw to indicate a crossover model.
I was initially drawn to this model when it was listed, not because of the black wheels ( although I did notice immediately the wheels were out of timeline) but because of the body colour shown in listing photos and the gap between the cab and the pump box. Whilst I have seen and studied models that are bent I had never seen it with a tapered gap between cab and pump box. So is the block deformed between the cab and pump box or have the walls of cab or pump box been pushed out of line?
If the bpw were added post factory I wonder if it was to rescue a model that had been stepped on. This could account for the bent area between the cab and pump box. The amount of force required to do this may have also bent axles and or damaged the gpw wheels.
This could then account for three new longer axles and 6 new 20 tread count bpw being fitted.
I would also suggest that the axle used for the middle axle is of a slightly different steel as it does not have the same patina and the axle end is not the same shape I consistently find on the roundish brace casting.
It is interesting that in 2021 we have now seen two 73a with bpw sold, one for 2500gbp on eBay and most recently another sell in the UK on February 5th 2021 for 1350gbp plus commission and vat. ( this was listed as having gpw

)
Both models shown were described as mint and boxed.
This variation is faked on a regular basis with around 10 a year being sold. My folder on such examples is growing by the month.
Bert