I think the original product was (is?) called 'Kwikfill' (not exactly sure of the spelling). However, white wax crayon, whilst not as hard as the original, makes a reasonable substitute. If it comes out, it's simple enough to just go over it again.
I wish I'd been in on this thread when the OP started it - I was away on holiday - as I have not lots, but certainly a fair few spare locking segments. He could have gladly had a couple. Over the years I've stripped quite a few No42's & 53's & only sometimes have I needed the segment. When not needed they just go into a little tray.......