Yes, I was just going to post about it myself when I found this. It's got a distinctive 3-pronged hull design. The details I have on it are:
Only ever saw it near Lave.
Packs quite a punch - at least beam laser level of damage.
Only ever saw it at the beginning of the game, when all I had was a pulse laser which wasn't enough to take it down.
Only managed to destroy it once (RAMMING SPEED!), no bounty or acknowledgement of the kill.
I suspect it was the 'cloak and dagger' ship (a cougar) from other version of Elite, but when I destroyed it there was no cargo (it apparently drops a stealth device you can pick up and use in other versions of the game). As far as I know the cloak and dagger mission wasn't in the C64 version. Edit: Found a picture! Is this what you saw?