The bolt doesn't have reverse thread. The key-word in unscrewing the bolt is using a perfect FITTING allen-wrench.
You can try to starle the bolt by hammering it a few times (at top of the corners in the allen bolt which will also cause the wrench to fit better again).
Mercedes uses loctite to keep that bolt in place and not risking any possible chance for the bolt to get loose.
Is there a cable in the 1980 SL? Our 1986 SL had a electronic speedo meter, but I can imagine that the older modeels did have a cable driven speedo meter...
with a minor 600+k