nec says "marked OFF position" if the t-stat comes from factory without one, will an after market (sharpie) marking suffice??

Nope. If the marking is absent, it is still very possible that the t-stat will let a minor amount of current through. This, for example, is how illuminated switched operate. UL will allow the "Off" marking-now, it wasn't always this way- if, indeed, the current really is 'off.'

normally the single pole thermosat dont useally have off indent or marking but to make sure you have off postion most common is the double pole thermosast do have off marking expect few specal thermosast they used without a marking because of " frezze guard " it will come on at 35-40 degrees range i am not sure excat number there

Merci, Marc

I think the reason that the single pole ones don't say off is because they often get used in two pole applications(cheaper). They would not qualify as a service disconnect in a two pole application.