You can see them on the rim in a hexagonal pattern in the picture below. Also present in the latest version of the Dragon trunk, but less clearly than here. They alternate in color from nickel to silver to nickel to silver... The silver cartridges have wider holes but seem otherwise identical.
What are they? Why are there two types? How do they work? If they are separation devices, are they just locks, or are they also pushers? Are they pneumatic, pyrotechnic, spring types... ?
And below is a picture from the matching top of the second stage. Again, this is an earlier Falcon/Dragon model, but the cartridges are more clearly visible here.
I circled the silver and nickel cartridges in matching colors. You can see they all have pins each with a hole here. These would presumably go inside the cartridges on the trunk.
Note that the silver rods are longer and thicker than the nickel green rods. They seem about twice as thick. Why? And could it be that least one of the two types of rods serves strictly to separate the trunk from the second stage, with the pin holes there to house a linear charge that would apply a positive force between the trunk and the second stage at separation?
You can see similar but beefier cartridges at the bottom of the second stage and at the top of the interstage, where they must house the pneumatically actuated mechanical collets mentioned in the Falcon 9 manual. (The collets unlock the stages before pneumatic pushers apply a positive separation force to push them apart).
Below is the top of the interstage during assembly. This is an earlier version of the Falcon 9. The newer version seems to have fewer collets and these seem to be housed together with the pneumatic pushers, but that shouldn't matter here).
...And below is the bottom of the second stage during assembly. There are two types of cartridges, or brackets, here, but I know what they are. The wider ones match the collet cartridges on the interstage. The thinner ones match the cutouts for the peripheral pneumatic pushers on the interstage.
Mentioning the interstage separation collets just because they are similar in some ways. Not suggesting they are the same thing (and clearly I have doubts, because I went to lengths to ask this question :D)
So (taking off from the very useful comments below) maybe the cartridges house explosive nut and bolt assemblies similar to the pyro devices used on the Apollo spacecraft? I've been resisting this option because pyro devices aren't testable on the ground (to use them or to test them is to destroy them)... And that would make them less attractive to SpaceX, even if neither the second stage nor the cargo trunk are recoverable. You'd still want to test.
But there is no trace of pneumatic cylinders, and the holes in the pins would seem natural only if you assume pyros... And... pyros have a proven history. Not one of them failed in the Apollo missions, if what I read is correct. And even if you can't test an individual pyro before flight, you can still test a batch of pyros from the same assembly line and know with high degree of confidence that the untested pyros on the rocket will work fine...
If SpaceX used pyros on the Falcon 9, and if the pyros were in just one place, this is no doubt where they'd be. I know they used pyros for the payload on Falcon 1 (it's in the manual for that rocket). But the payload fairing and Dragon trunk are different things. The payload fairing doesn't have those cartridges at the back, and the connection is to a conical payload adapter on top of the second stage. That payload adapter isn't used for the trunk...
A view of the mating cartridges on the second stage and the Dragon trunk. Newer version of the Dragon. Must be the separation mechanism, but what type? Pneumatic, pyrotechnic? Locking collets, pushers, both? Pyro bolts and nuts?