I have a 10K spindle in my 9” Workshop headstock.

I do use 6K/3C/3AT collets.

I do not have a speed closer. I do have 3 handwheel closers.

The 3C and the 3AT have essentially the same threads but the 3AT is shorter, a bit larger in diameter and has a different taper than the 3C.

I have two POT collets for 3C.

I see no reason you could not make a tube for the 3C to fit your speed closer.

You would need a taper adapter sleeve for your lathe, to take 3C if you do not have one and a Nose piece if you do not  have one

Now a once in a lifetime offer.

I have a brand new 6K pot collet. Never used. It’s a big one, 4-1/2 OD.

I will probably never use it. I have had it since about the time SB had their going out of business sale.

I would be willing to part with it. I am getting old and need to start clearing stuff out.

Not inexpensive but if you are interested contact me off line.

I also have 3 6K SOFT steel collets; a 1/8 a ½, and a 9/16” (9/16 could be 35/64. All are unmarked.

They say Modern, 6K, soft.

I think you could just turn a shallow 1” ring in them. The OD seems to be 1-3/16 (1.178), the key is 0.080 deep.

The ring might break through the key.

Perhaps one of these would do better; I would guess the 9/16 one could work.

These would be cheaper.