Formulas don't count merged cells

Hi there,

I'm sorry if this is a basic problem - I can't seem to find exactly the solution I'm looking for...

I want to count the number of times we have sold models manufactured between 1965 and 1980. Unfortunatly, some models have had more than one owner... As soon as we have a one model sold several times, we are using merged cell, but then the formula stops working because of the merged cells (see attached sample document). Hopefully I'm making sense... Can anyone give tips on how to solve this issue?

Thanks in advance,