Thank you Morten. I had never heard of BSP thread but it seems to correspond to my existing fitting. My car actually has an RCR top plate, not a RBT, so it might be different. I pulled the fitting and it measures 25.9mm OD on the threads, so not quite 26.4, but probably close enough. It does seem to be 14 TPI though which aligns.
Luckily BSPP (parallel, not tapered) to NPT adapters seem to be available, and then NPT to AN adapters are available. I think I'll just order one and see if it fits. Thanks!