How do I tell a factory M4 extractor spring from a factory AR15/M16 extractor spring if it has no associated colored insert?  I have read the number of coils is different?  How are they counted?  Does anyone have a comparison picture?

Submariner,
I got up out of my comfortable chair to make this photo for you.

The extractor spring on the right is a new Colt M4 spring as purchased from SAW Sales. It appears to be  more "open coil" ( space between coils is slightly greater ) than the other springs shown for comparison. It also appears very slightly longer that the RRA standard. It would be hard to identify with certainty a separate spring of unknown source as a "Colt M4 spring". The black insert is of no value in this process.
The best buy in extra power extractor springs are the Model 1 Sales spring on the left. About $1 and works as well as the Wolff, SAW house brand, M4 or the others I have used.

Quoted: How do I tell a factory M4 extractor spring from a factory AR15/M16 extractor spring if it has no associated colored insert?  I have read the number of coils is different?  How are they counted?  Does anyone have a comparison picture?

What I was told by Wolff is that the spring tension is determined by wire diameter and coils per inch. This was in the context of a question about how to make a spring they did not stock.
FWIW
YMMV
DFB

www.efunda.com/designstandards/springs/spring_introduction.cfm

This place has some good info on spring design. One of the things is the wider distance between the coils that makes the spring stiffer. Going by what I have bought I have not been able to find any difference in the Colt extractor springs other than the stiffer insert. Could be the type metal used? More coils does not do what most think it does.

.