As a writer my manuscripts are often page numbered "X of Y" because the software does the updating of both numbers automatically as I type. 

However I've never seen or heard of a builder numbering instruments "X of Y".  That implies that he/she was only ever going to build Y number of instruments either in total or of a particular model.  

From what I know they are a USA company, and was building dulcimers in 2009. He built enough dulcimers so that there should be some recognition, and the build was good. Not just a hobbyist. I just restore an hourglass that has a number 18133 but I doubt he ever built that many, so it may be 18 of 133 or 181 of 33 or etc. The date on the dulc is 01-09.