20-count

Dear experts in historical linguistics,
What is the originof the 20-count in French, Danish, older English
(score), Old Norse (skor), Insular Celtic and dialectal German (steigs)?

I've been doing some research in the literature but I figured I get
better answer here. E.g the majestic Gvozdanovic volume "Indo-European
Numerals" have but little information on it. The chapter on Romance does
have some interesting information, suggesting that the 20-count for French
comes from English which in turn might have got it from Old Norse, with
some evidence and counter-points. But the Celtic chapter has almost no
information on theories of the origin but makes it clear that the system
probably does not date back to proto-Insular Celtic. The chapter on
Danish only mentions the ultra-peculiar Danish system in passing.

grateful for any information,

Harald