mcb66, not true. You can. Mark I, II, and III all use the exact same bolt and bolt parts, including the firing pin stop / return spring base that prevents it from impacting the chamber (the original Mark I manual shows that part going in upside down; it doesn't fit that way). I know way more about these guns than I would like to, thanks to having to troubleshoot my father's Mark I, which is now wearing a Mark III mainspring assembly, recoil spring assembly, and firing pin. Fitting those parts to a 1972 gun was fun, and required a bit of filing to both gun and parts to account for tolerance differences.
All modern rimfires that I know of can be safely dry fired. It is only the older ones that had a real problem. If you pick up an old classic, you should assume they shouldn't be dry fired.