After two earlier unsuccessfull attempts, this evening I finally found a Hella pre-glow time-relay listed under 'body-electrical' on Fastlane - for $193.50.
It supercedes the earlier designs and replaces both W123 240 and 300 diesel glowplug relays.
This upgrade part operates the glowplugs for up to 3 minutes after engine startup and appears to require replacement glowplugs.

Happy Motoring, Mark

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DrDKW