Bear in mind the disclaimer that it must be 25 years ago since I had any dealings with the Rowatt gearbox.....
Yes it was based on a planetary gear set, which came out of an overdrive from a UK built car, for example BMC Westminster. That was more or less the only production parts used, the rest being special to type machined parts.
For its time it was a cheap alternative to a Lenco, which is a under drive, but one inherent problem was too much drag, which robbed power and also made pushing back, reversers were not compulsory at this time, hard, especially for Dave Coles who coupled two together to make a 3 speed, hence the trolley he made to lift the rearend after the burnout!
That’s my brief take on it, although there are at least two people on here that had more ‘hands on experience’.
Herb