Can you take the watch to a watchmaker and have him open the case? It would help to get all the numbers inside the case lid, and also on the movement. With them, we can identify the model and the movement, and most likely the year.
I have little doubt this is an 810A, but when "IWC" was on the dial most of these models were renamed as 1810. But the 1810 also had a date, and also normally were not with blue dials. See, for example, this reference from the 1975 dealers' workbook:

There also were similar models with Cal. 89 movements, although with a different reference number.
Part of the problem is that the mid-1970s were a period of upheaval for the Swiss watch industry, and IWC as well. A lot of dials were placed on models, some with different movements. It makes things interesting ;)