To begin with, over the years, and possibly still, Selden has made several different series of booms and boom fittings. I recently redid a 4 year old boom, to add more purchase to the outhaul. It was very different than the roughly 15 year old Selden boom that I helped with some years before that. The modern boom has a series of tracks and specialized plastic cars inside the boom with the sheaves for the outhaul and single line reefing systems. The cars ride in raised grooves at the top and bottom of the boom. They are configured so that they can bypass each other without interference. But because of this configuration, it is critical that the cars be installed facing the right direction and that the lines are reeved perfectly. If not they collide with each other rather than bypass each other. The 4 year old Selden boom that I worked on was assembled wrong so that the outhaul car would collide with the shuttle block for the single-line reefing and would jamb at some point and randomly not work properly at other times.
The reef lines and sheaves dead end on removeable pins. The one at the gooseneck is accessible without removing the gooseneck. On a a 2:1 outhaul or 4:1 outhaul, the outhaul dead ends on that pin at the gooseneck. On a 3:1 or 6:1 outhaul, the cascade dead ends on the pin at the gooseneck and the outhaul dead ends on the car inside the boom. You can see the configuration of the cars here. Unfortunately it does not show an end view so that you can see that the cars (sliders) are shaped so that there is a car that faces the exterior of the boom and a similar car that faces the interior and you need to use the right car for each track.
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The new booms use Allen screws (aka socket screws, AKA hex key screws) to keep the boom end fittings in place. (I can't tell whether from the photo whether those are rivets or allen screws.) Once the allen screws are removed, the boom ends slide out easily. You need to be very careful when you slide the ends out since the pin that holds the sheaves is not captive and can slide out making it easy to lose the pin or the sheaves over the side.
While I was not enamored with the overall engineering of this system, the nice thing about this system is that, if reeved properly, the lines cannot get twisted up inside the boom. That means that when the boom is being assembled, you need to be extremely careful to feed the lines without twists and without catching other lines within the boom. This takes some care and concentration since you are holding a series of loose parts in your hands as you are reeving the system. I also do not love that all of the parts are proprietary and, based on my experience, it can take quite a few weeks to get even a simple replacement part such as a sheave or one of the cars.
Another issue with this system is friction. The sheaves only accept a relatively small diameter line. We ended up using a dyneema cored line and stripping the cover off the part of the outhaul that is always in the boom which helped a lot. But that small diameter line tends to slip in the clutches and so we are planning to bulk up the cover at the clutches.
I will also note that SanderO's boom configuration is probably the one that went out of production in 1991 and Mark's boom configuration is probably went out of production in 2007 or 2008.
I am not sure whether or not this helps,
Jeff
