I'm going to add to this thread but probably just create more confusion.
I have a "Shopsmith" branded molder head, shown in the photo below.

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It differs in some respects from the one currently shown on the Shopsmith site. Here:
Speciically, mine lacks the Corob name on it, and also doesn't say MKHD50 on it. Nor does it have the three holes in the face of the head.
Yet the measurements of my molder head match exactly those shown on the Corob site for their MKHD50 molder head. Note that this head is described on the Corob site as a "Delta Style" head.
Now, today I received a long-awaited 3 bead molding knife from Shopsmith. It came in a plastic envelope with Shopsmith's part number for the "old" style knives, which I ordered.

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But the original packaging says "Corob" (no surprise, we know they make 'em) What is interesting to me is the Corob packaging has the designation "HD" in the upper right corner. If you dig around on the Corob site, you wil find out that HD is their designation for their Delta Style knives.
Now, I've got some other knives that I got when I acquired this molding head (second hand) and I acquired some others from Shopsmith about a year ago. Some of them do have the HD designation and some don't. This one doesn't say HD.

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But this one does.

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And the knives are different thicknesses. The HD one I received today is .125. Older non HD knives are .129. They also have different size bores, which I think you can see here.

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I have used both kinds of knives in my molder head but now I'm not sure what I should be using. The Shopsmith website says to use the new thicker style knives with heads made after 2011. I don't know when my head was made. I just know that it is physically like the new head in critical dimensions even though it has some cosmetic differences. I wish Shopsmith might provide some better information.