Hi Andrew,
I can only give my experience based upon my Dickson Toolpost and the toolholders I've bought after-sales which were supposed to be compatible.
My toolpost was made by A&R Precision and I have bought toolholders from A&R (and Lynam only difference is that the new ones are blackened), Myford and Chronos (Soba). They are not all equal.
The A&R and Lynam ones are by far the best IMHO in terms of functionality. The screws and fittings are good quality and the lower shelf is the thinnest. With the A&R toolholders I can accommodate 10mm tooling on my ML7 and still have on-centre.
The next best were the Myford toolholders. They were more expensive than the current Lynam prices but fitted my toolholder straight off. The screws are good quality but the lower shelf is slightly thicker than the A&R/Lynam – meaning I can only just about get 3/8" tooling on-centre. Certainly not 10mm.
The worst were the Chronos (Soba) toolholders although these were the cheapest at the time I bought them. The screws were all soft and I ended up replacing them all One screw was off-line and was a bu**er to retap. The slot for the clamp was too thin on all holders I bought and each holder required fettling with a grinding wheel in a Dremel to get them to fit. On these the lower shelf is very thick and I can only fit 8mm tooling in there and get it on-centre.
I hope this helps
Jon