I'm in the process of cleaning up an old 1968 (as date-stamped on the taps)
stainless steel sink + drainer + taps. I'm very happy with the result, come
up very well and the sink depth is much more than you get nowadays... and it
has only cost me a new sink waste & plug...

However, there is a black tar or bitumen like square pad stuck on the
underside of the sink (and another under the drainer) that has mostly broken
away. Another s/s sink I'm chucking out had a similar thing.

Anyone know what these are for (are they to deaden sound?) and if/how to fix
up?

Thanks,
Paul.