Nigel
They are a bit "rocking horse excrement" second hand, Walther front dovetail is different to all else, you can get new versions such as Centra in Walther size but not really looking right on a classic.
We are old school and always order the traditional type that takes metal elements on our new rifles (all Walthers) and they will supply if you ask. Walther do list the 18 mm old style ones, just look on their website in Zubhor, (parts), I order through MEC or collect when I'm over there.
Best of Luck
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?