Hi, Steve-O and Kemp. Well, yes and no. We've actually worked out the hinge mounts (not easy!) and rubber supports for a CB550 and mounted it on a frame. However, there are a couple reasons why we haven't moved forward on the project.
First, without redoing the molds for the foam and seat pan, the inner rear fender fouls the existing seat base and molded foam. Due to setup charges, modifying them would be a costly proposition. The only alternative without a seat pan mold redo is to trim the CB500/550 inner plastic rear fender at the top where it interferes with the seat pan and we try to never make anyone modify their bike unnecessarily.
The second issue is that the early CB500/550 peanut-styled fuel tank must be used due to the length of the current seat and upholstery design. Our seat is an exacting copy of the original Giuliari for an early CB750. The late CB550 tank, like the CB750 K7-8, has a longer tank. Fortunately, the earlier tank is a bolt-on to the later CB550 frame. We cannot, will not make new upholstery due to the astronomical cost of the die used to imprint the perfect Giuliari embossed waffle pattern.
BTW, the combo of the early tank and Giuliari-style seat works very well on a 500/550. Very fluid lines and highly integrated design. I've attached a few photos of our prototype CB550 seat fitted to an old frame and junk early tank as well as of a completed CB550 with a Giuliari seat taken from the Web.
We'd like to make a seat for the middleweight but the only way that will happen is if we have, perhaps, six or more confirmed orders with significant deposits and if the buyers understand the need for the early tank and have the patience to wait perhaps two months for the new foam and seat pan molds to be completed and their seats built. So far, we've been reticent to even ask.
I hope this answers your questions and that you have a sense of what we go through to make as nearly perfect a seat as humanly possible. If anyone has any additional questions, I'd recommend that you PM me. Thanks for wading through this posting. RR