If it's what I think you're referring to, there should be two of them! And they're small, only about a couple of inches long... right?

If that fits the description, they're a pair of pressure limiting devices for the rear brakes on the two separate brake circuits. These devices prevent maximum pressure developed in the brake circuits from being transferred to the rear brakes, to prevent rear wheel lockup. It's considered, and required by law in Europe (TUV regs) that front brakes always lock up before rears lock up, thus maintained a safer posture (facing front) in a brake-induced skid.

Some call it a proportioning valve, but that's not quite right. My old 164 had a real proportioning valve (and it was adjustable), but this (on the 240s) merely limits the amount of pressure to the rear brakes.

And the reason why there are two of these (I'm wondering why you only saw one) is that you have two independent, "triangular", brake circuits. Each separately controls both front brakes and one of the two rear brakes -- thus each rear brake needs its own, matching device.

By the way, what I wrote is applicable to pre-ABS (pre-'92) 240s. With ABS, the system is altogether different: no separate circuits, no pressure-limiting devices, etc.