Hey Greenlake, they do serve somewhat different purposes.
In general, cam cleats will tolerate a fair amount of horizontal misalignment and some lesser degree of vertical misalignment. In this application, as the cleat and the block on the jib car are at different heights and not on the same plane, the sheet always enters the cleat at an angle. Additionally, adjustments to the jib cars cause the horizontal alignment to vary, although this is somewhat less critical. The wire fairleads on the load side direct the sheet into the cleat with as little misalignment as possible. You can see the sheet does change angle in the picture as it passes through. This is more noticeable under load - when the sheet is pulled taught the ratchet block can swing up higher and the change in angle becomes a bit more pronounced. Honestly, with the low loads on the sheet (there's what, 50 square feet of jib out there?) and the surplus holding power of the H150 cleat and decent sized line, this is largely unnecessary. Additionally, they aren't particularly rigid. If the sheet was under such load that the slight misalignment was testing the holding power of the cleat, they would probably just flex and deflect. However they are inexpensive and have some small benefit, and as I already had them (I ordered a variety of hardware and fittings for this project) I installed them.
The plastic fairleads on top of the cleats have no effect on the load side. They are meant to help guide the line into the cleat on the trim side. With them in place it is possible to cleat the line at a fairly significant angle - say from the stern of the boat. They also prevent the sheet from coming completely clear of the cleat (not really an issue in this case as the wire fairleads would prevent this anyway).
With this setup, there is no need for any karate foot action as all angles and leads were selected to make the jib sheets as easy to cleat, uncleat, and trim as possible for a 5' 5" female sitting on the rail. My 6' limbs can make do either way, but the sheets cleat at somewhere around waist level and uncleat below shoulder level - an easy reach for about anyone. I didn't want anyone to have to take their feet out of the hiking straps (still yet to be installed of course...
) or lean down into the boat to cleat off. During rigging the sheets are pulled through the fairleads and cleats and a stopper knot is tied (they're quite long). If the need arises to blow the sheets they can be yanked upwards and allowed to run or even thrown into the center of the boat with no chance of losing them. The issues with the original jib car mounted cleats are pretty well known and have been discussed in a number of places on the forum, but it was immediately evident that if the breeze was up it took pretty significant muscle to uncleat the sheet. It would often become tightly wedged and acrobatics would ensue. One user's comment about how an inability to blow the jib preceded nearly every Daysailer capsize he had seen made a lot of sense to me. There was also so much drag coming through the bullseyes on the original jib cars that the crew often had to really fight to sheet in the last 6-12" after a tack.
This isn't a perfect solution for everyone or everything, but it's been effective so far. If I tear a cleat out of the centerboard trunk or end up swimming because of it I'll certainly share that as well. I have a few more projects and updates coming this spring and will collect them all into a build thread.