Model number problem here.
The Ruger standard model made before 1982 (I assume hkmp5sd is correct) was not the Mark I. It was just the Standard Model. The Mark I was the target version with Micro sights, available in a 5" bull barrel, a 7" heavy barrel, and (IIRC) a 10" barrel.
When Ruger made the change (as described above) all the models were designated as Mark II series.
Jim