Hi,

I have several cars of the 240 series. My cars range from 1978 through 1993 that have he M46 and the M47’ that have a fifth gear overdrive selection.

The M46”s were used until the M47 came about
The M46 used a “J” type Laycock hydraulic overdrives in the USA.
I’m not familiar with the “P” type except that it’s either older or used between European countries.
The P and J are both British and used in Mikes Garage (MG) cars of similar weights.
The 1800 was also a light car when compared to the Volvo 100/200 series family targeted consumers.

The 1800’s production ran along the same years and I can only guess were mostly M45s.
Even in 1978 the overdrives were an option in the USA.

There is convicting information
There is no brake system like on some older vehicles that actually grabbed the driveline shaft with a banded clutch material. Old school truck technologies.
Something like the term “brake” seems off
The “J” uses hydraulic pressure principles through an electrical solenoid.
I’m concerned that what maybe called a brake hub inside the overdrive units is really a clutch cone assembly.
I don’t know how the “P” engaged the, something like a planetary gear train, to obtain a 20% overdrive ratio.

I see the same post you made in the 1800 column or tab so you are asking in right places.
Trick is identifying how much interchangeability thee might be.
I think I have read that the dashes mean revisions in production across its vendors to keep track of things internally to Volvo. Some might be nothing more than price changes and the age of inventory and support evaluations.

Someone in the right circles will be happy to grab them up.
Maybe the shop out in Torrance California can help you.
Eric hangs out on the BRICKBOARD with great information.
Try this
http://hiperformanceautoservice.com/

Phil