I really don't think it was thermal paste, the deposit was only in and around the mounting hole in the mica washer. There is an identical transistor nearby on the board so I unbolted it and checked underneath in case there was an incipient problem, it was completely clean with no sign of any paste ever having been used.
Whatever it was was less substantial than heatsink compound, if you know what I mean. It more or less disappeared when wiped, whereas my experience of heatsink compound is that it is incredibly persistent and the smallest amount gets everywhere. This is a particularly unpleasant characteristic of the original heatsink paste, silicon grease, as it is bioaccumulative.