posted 03-05-2012 07:03 PM
I did some research on this topic some time ago, planning on writing a page for my website (but hasn't quite progressed that far yet).
I've found that there's no real consistency with respect to what they're called: I've seen them called (slow, soft, or controlled) release (mechanisms, pins, or rods). This certainly complicates keyword searches!
Whatever they were called, they were used to "reduce transient stresses resulting from abrupt disengagement of a vehicle from its launch stand." This dynamic transient could be enough to damage the vehicle.
To prevent such damage, they put a number of dies on the S-IC's thrust structure which engaged pins mounted on either side of the holddown arms. After the holddown arms released the Saturn, the rods were stretched through the dies, slowing the initial acceleration over the first 6 to 7 inches off the pad.
As you mentioned, the number of slow release pins varied over time. SA-501 (Apollo 4) had 16 pins; it took 1.17 seconds to free the vehicle from the pins and limited the transient to +/- 0.2g.
The amount of force necessary to clear the pins was larger than anticipated. For SA-502, they reduced the number of rods from 16 to 12 and started lubricating the rods. It then took 0.54 seconds to break free, resulting in +/- 0.4g. SA-503 also used 12 rods.
Documentation after SA-503 is more limited, but I seem to remember an indirect reference on one vehicle (I think it was 506) indicating that the later vehicles used only 8.
Information on the liftoff transient loads are available in each missions' Flight Evaluation Report (available through the NASA Technical Report Server).
I never found any photos of the full and complete mechanisms. Photos of the portion of the die attached to the S-IC in the VAB are fairly common (once you know what to look for); a very good one is about 2/3 down the page on this blog posting by Jonathan Ward.
Diagrams of the mechanism are much easier to come by. Some of the better ones can be found at page 161 (page in PDF) of Mission Operation Report: Apollo 8 (AS-503) Mission, page 366 in the PDF of the 9th Aerospace Mechanisms Symposium, and page 70 in the PDF of Symposium on Transient Loads and Response of Space Vehicles.
Once you know what to look for, you'll also notice the dies on the S-IC's thrust structure in launch footage. If you happen to have the Mighty Saturns: Saturn V DVD set, there are plenty of clips which show the dies on the S-IC.
I have the original Spacecraft Films (i.e., before the Fox distribution deal) Mighty Saturns. Among the Apollo 8 footage on Disc 3 is a film clip which shows a pin being extruded through its die. In the frame, the pin is located right below the tail service mast. You can watch the pin get longer and longer, until it is lost as the dislodged frost obscures.
If you've read this far and are still interested in more, drop me an email and I'll send you a copy of my notes and an (even more) exhaustive list of diagrams.