Need a detective here...

[Truzi]

My thought, if there is no date (or not enough detail on the negative), is to see whether the state had any changes to the number of characters on the plates, or when certain letter/number standards were set. It might be a long shot, but the 1 letter - 4 number format may or may not help date it.

You could also comb over other family pictures, and if you see the car again, there may be other indicators of date.

 

[DWThomas]

Pennsylvania is most cool.

Yes -- interestingly, except for the car you can find similar scenes in some of the more remote areas even today. That's an interesting archive the OP has. I discovered my parents apparently dumped all their negatives somewhere along the way -- and their collection of stacks of prints has transferred ink from ballpoint writing on the back to the front of the next print in many cases. Oh well, many were "another green wooded mountain" probably more meaningful to them than me! We do have some 8mm movies transferred to video tape -- somewhere -- guess they should be going to DVD soon!

 

[37th Exposure]

I assume the film has no edge markings? Otherwise the manufacturer could place the time of manufacture from an emulsion number or even the typeface. Kodak changed the style of the markings every few years for example. This could give some corroborative evidence as to when the photo was made.

 

[AgX]

Basically base material also could be a hint at production time of film.

But we must not overlook that film production time does not necessarily coincide with time of exposure.

 

[37th Exposure]

True, no one knows how long after purchase the film was actually used but since conclusive evidence is not available, date of sale/ manufacture could add another piece to the puzzle.