The message
from Dave Plowman contains these words:

In article ,
David Holgate wrote:
Thanks to those who offered advice. The designer has now been in touch
and explains that the active devices (one or two when placed in contact
with one another) should be secured via a mounting screw inserted from
the outside, through a mounting bush and then the heatsink (with a
suitable thermal pad on the other side. This gives the right insulation
and thermal transfer.

But isn't this the standard way of mounting a device with a 'live' tab?

Probably is--this is the first time I've assembled such a kit, so I'm
learning as I go along. :-) The kit just said in the standard way, with
no further details given.