Wikipedia had the same, "The Keep on the Borderlands was first printed in December 1979 and published in 1980." The Wikipedia source was Shadis #29 from 1996. Just edited it on Wikipedia as well.
The Holmes Basic 3rd edition of the rulebook may have been printed in Dec. 79, but the box and other contents look to have been printed later. Before printing on demand, a larger run and storing some stock was cheaper than multiple shorter runs. The alternative is that they did do multiple runs, but kept using the same plates and skipped the expense of changing only the title page.
Funny enough it was Frank retelling the story of catching the "glitch" in
B2 that got my brain to snap onto this glitch. Glad it is fixed now.