Michael,
I can answer some of your questions:
1) are you able to VISIBLY IDENTIFY the difference between a stock and (let's just say) a GUARDER recoil or hammer spring? i ask because i have about 6 or so springs that i am unsure of the manufacturer of. some examples are below.
perhaps there is a test to see the tension? perhaps color or length (although almost all identical, i can see some SLIGHT variation, it could also be my aging eyes...)
From my experience, Guarder has two kinds of hammer springs: One which is sold by itself, the other which is sold with the recoil rod/spring as shown here:
and here:
Based on your pictures, this is my guess:
I can tell you that they are not Shooter's Design hammer springs though.
I had the best results with Shooter's Design hammer springs, since the tension on the springs don't weaken over time. Guarder also has really strong hammer springs given by the angle of the two prongs, but whether the tension will be maintained in the future is uncertain.
2) have you ever found doubling up a recoil spring to be beneficial? this is simply out of curiosity.
Yes, you can do this to increase the ROF by using a second stage spring in series with your primary recoil spring. Or you can use a full length recoil spring with a 1/2 to 1/3 of another spring. However, you will need the plastic "C" clip to fit between the springs. The more springs you put, the more you short stroke the gun will be. However, if you put too much, your slide will not lock when the magazine is empty, and you may have issues chamber the next round. You can use one of the aftermarket upgrade guide rods (Shooters Design, Guarder) and use two springs fitted between the "C" clip. You can also add buffers, or if you really want to go extreme, a lot of japanese racegun users insert a tube over a portion of the recoil spring to increase cycling speed. Here is a picture of the two stage recoil system for glocks. As you can see in the picture below, the spring is short, but very stiff which helps to recoil the slide back to battery. You can put your stock spring inside the rod to go along with this second stage spring.
