The factory changed to the '74 model year (IB) cars after the August holiday break.
In that first month I do not know of a September built car that didn't have RS arches fitted originally.
I believe that mid-October is when the RS arches ran out.
Any original survivor cars I’ve seen that were November 1973 built, appear to be configured in the SC version.
Therefore my modest research of the build dates (in conjunction with that of the chassis numbers) leads me to believe the stock of RS arches was exhausted in the middle of October. That is informed speculation but it’s getting impossible to define the crossover point any more accurately than that.
I think the RS tunnel reinforcements also ended around this time but I’d love to hear from anyone with a car configured as such but built later than mid-Oct ’73.
It’s quite possible the arches and tunnel reinforcements ran out at different times but you’d imagine the factory would have ordered similar numbers of both parts? I don’t know.
So, whilst some of this is conjecture, I'd say - as best you can 40 years after the event - that the changeover can be pinned down to within a couple of weeks production and amongst a spread of less than 50 or so cars.
In light of this (and along with the remaining evidence of my cars tunnel reinforcements) I am therefore quietly confident that my car should have RS style arches and I will rebuild my car accordingly.
