Can anybody help me with details of the Sanwa SCS401T and SCM401R 40KHz
ultrasonic transducer?

This part is available from Farnell as 143-736 and 143-737, and Maplin as
HY12

I have a matched pair. One marked 40T which seems obvious as the
transmitter but this doesn't show any polarity. Niether connection is
connected to the screen can. Is polarity important for this transducer?

The other must be the receiver but is unmarked, however one pin is obviously
the ground as it's connected to the screened body.

Thankyou.

James