Here are Higgins' photos:
I assume I would not try to pull that down below the cars to the level of the boom, as I think that the wind would put a lot of side pressure on those ?
Yes, they're slugs, not cars. Making up a dogbone (two rings with piece of webbing between them) that goes through the cringle would help, but, yes, you would want to pull the cringle (big grommet) down to the reefing hook if you don't have a dogbone.
I have a track with pulleys at the booms end, that looks like the Harken reefing system. But there are 2 double pulleys near the front, and I have no idea what they have been used for ?
(They are NOT in line with any cringle or pulley at the mast or deck ?
Typically, if you have a single line reefing setup, you'd have blocks like that setup—and that is what they are probably intended for. You'd have the reefing line run up from the boom, to a block at the aft end of the boom, forward to the first aft-most double block, and up to a block attached to the sail at the reefing cringle, and then down to the forward most block and then aft to a cleat. You've got two cleats on the side of the boom and probably only two reefs in the mainsail.
But the forward most blocks look like they're designed to have a line come down and then go forward... not back aft to the cleats, at least from what I can see in the photo. Do the reefing lines have blocks at the base of the mast to run them back to the cockpit?? If so, that would explain it.
I'd point out that double-line reefing systems, although they require more hardware, are generally better IMHO, since they allow you better control over sail shape with less effort—since you can tension the clew and tack of the reef separately.