I'm looking at a K98 bayo with 43asw on the blade (E&F Horster & Co) which also has a WaA518 waffenampt on the blade. I'm being told that waffenampt on the blade is unusual and makes this K98 scarce.
True?

yes... this place for the stamp is scarer than other ones. but servel exist with this stamp.

Here is one, matching numbers.
I suspect thoses Horsters to have first been rejected, then reworked or refinished, then accepted and proofed on the blade. Cause thoses bayonets with WaA519 on blade (43fnj or 43asw) always have a less good finish than thoses without WaA on blade and show a flow somewhere or a default.