In the picture Ron posted of the Table Riser Assemblies, which looks suspiciously like mine, there are two different main piece castings. The one on the left is the same as the one from the ad and the same as John's and Bruce's. The one on the right is a bit different as it has a stop for the cast collar that is on it and is the same collar as the one John has. I recall seeing one somewhere that has the collar similar to Bruce's square one but with a slightly rounded back. I have 2 of the one on the right that are the same. Both came with damage to the lock knob. One with a broken off threaded brass stud and the other with what looks like 2 drill holes in the face on the knob. This was the less common Table Raiser Assembly made for the Model 10's.
I believe the rarest accessory for the Model 10 is the lower belt guard. I have yet to see one for sale either on it's own or with a Model 10. It is followed by the Lower Blade Guard. The rear fence lock for the original 10E fence is not very common. The key switch and the two step pulley are harder to find than more common accessories.
Although included with the purchase of a Model 10 I believe these next two items are worth mentioning. The Jacobs 33BA chuck is still available today, but the use of the Jacobs 33B Chuck with the Chuck Arbor P/N 102-47X was not available for very long as the Drill Chuck was replaced by the new Jacobs 3326 Chuck. The original Upper Blade Guard with the slots in the side and it's splitter was not available very long and was discussed in the thread New to Me Model 10E Serial Number 1077
Finally if we look at the picture of the Model 10 from the original typed manual there are several differences between what is shown there and believed to be a pre-production Shopsmith and what came out with the first Model 10E's. The 4 ribbed Sanding Disk, the saw blade arbor and others come to mind.
Getting to this point I really can't think of any hard to find Mark 5 accessories. There are Mark 2 and VII items along with the Sawsmith items that are hard to find. Does anyone know what Arbor P/N 517118 was for? Did you know the SawSmith Table Saw had a different Molder Head? Bruce you are not allowed to answer that one.