The OP asks what size people have on what lathe. Not directly helpful to him but I have three on different lathes.
The Myford Speed 10 has a genuine Multifix AA, £41 from a local tool dealer, complete with 2 holders. I fell lucky. Since fitting it, I've bought extra holders from Rotagrip, Paulimot and Pewe, the last unhardened unslotted to machine for particular purposes. All have fitted fine.
The Harrison 11" (better known as an L5a) has a size A from Rotagrip with extra holders from them also. This was the first one I fitted, to replace very unsatisfactory Dickson clone. Main issue with the Dickson was that extra holders didn't fit, regardless of where they came from, including the original supplier! A minor issue, at least for me, with the Dickson was the far right corner of the body fouling the work when partially rotated. I know the theory is that the toolpost is pinned at 90 degrees but I struggle when using hss tooling, as opposed to insert tools. I seriously considered cutting a corner off!
Most recent fitment was a genuine size C on the Harrison M400. This was s/h from Home & Workshop with 4 holders, albeit 2 for boring bars. Two more plain holders came from the same supplier and I've bought a few more from eBay vendors ( UK based, selling used holders). I may have been lucky, as opposed to cautious, but all have fitted fine.
I'd also like to pay tribute to Home & Workshop. I enquired about a C post at Warwick show several years ago. (They had a D on the stand). The following spring I went to Doncaster to be met with "Are you still looking for a Multifix? We've got one put aside in case you are." Pretty good going, considering I only buy odd bits and pieces from them at the shows and hadn't asked then to reserve a C for me.
Usual disclaimer, with regard to all the traders I've mentioned.
As you will gather, I do like Multifix QCTPs. I've got to them gradually and to be fair to the Dickson type, I did use good clones for several years on both a Super7 and a Kerry AG23
Mike