Grasshopper,
Time for a short history lesson. Way back when, before WWII, the military (and later the entire federal government) was busy making mechanical drawings of the parts they wanted built. depending on what size the finished drawing was they could range from size "A" all the way up to size "F". Thus part D 6528291 refers to the receiver of a Garand rifle, on size "D" paper. Later, the Federal Supply classification was added to create the 'Federal Stock Number' and even later an additional two digits were added to create 'National Stock Numbers' so things could be sourced from allies (two way).
Now, on to your specific need. From the link you provided, what you are looking for is
NSN Number: 1240-00-768-8875
Part number: 7688875
the part number (7688875) is the drawing number, in this case for TAACOM (Tank and Automotive Command). You can request (at cost), copies of the drawings from TAACOM, if they are not classified or otherwise restricted (and they probably are). There are probably some hoops to jump through - lots of them if the part is restricted or otherwise classified, but this should put you further along than you were. HTH.
