Once again....... In the real world we would just polish or polish out the small gouge and re-use the plunger. This gouge is caused by the use of a Mk2 backsight leaf* and in any case, the gouge will have no bearing - if you'll excuse the pun - on the operation of a Mk1 backsight. Just polish that surface and get on with the bloody job..........
* just in case a cleverarse is about to jump in, remember that on the No5, the fabricated backsight is a Mk2