Brinn does not offer it, but you are correct, Bert has the two speed, internal clutch transmission used in late models and modified. They sell it with a sprint car engine plate, bellhousing, flywheel/ring gear and on board starter assembly. There are quite a few in use mostly in dirt cars, they haven't been available lately as the foundry that made some of the casting went out of business during Covid. Downsides are cost,weight, the ring small ring gear diameter makes it really hard on the onboard starter, and you have a relatively short driveline due to the length of the whole transmission assembly. It's a really clever design all in all.