Very short (sub 30") a body speedometer cable? '72 Demon

During the course of replacing the a/c vacuum switch in my Demon I had to remove the gauge cluster.
When I attempted to disconnect the speedo cable from the back of the gauge cluster the 40 odd year old brittle plastic end just crumbled in my hand. Doh!

I've been looking online and the only speedo cables I can find are listed as some 60+ inches long. Whilst I haven't removed the cable from the car yet, there's no way I need something that long. I doubt it'll be even half that long?

The cable I have passes through the bulkhead and is secured to a large round canister with a 1" nut. There is a second cable on the bottom of this canister that goes off, presumably, to the transmission.

Does anyone happen to know the part number for this short cable or, indeed have one hiding amongst their stash?
What purpose does this canister serve? Is it there to prevent fluid from the gearbox traveling up into the gauge cluster via capillary action, perhaps?

I've attached a picture to illustrate the canister.

Thanks in advance,

Jon.