I don't think Nova Scotia Power used NEMA heads after the late 1970's. I also don't believe they used any in other than MV. I would guess 175 watts would have been the standard, but maybe some of 100 watts for smaller roads. From what I've seen, NEMA heads were a minority with them. Most installations of that era would have been Westinghouse Silverliner or equivalent, and ballast-in-arm units for individual customer rental. Some smaller streets used
these , however.
All that being said, NEMA heads were (are) made in HPS so it's possible yours is one of a very few installed by NS Power. Another possibility is that it was converted to HPS by an employee, back in the day when they had time and staff to do such repairs at every office stockroom depot.
Like others have said, pictures are worth a thousand words. When we can see what you have, we'll be able to tell you with more confidence what you want to know.