You don't even need an extra yoke...none I have ever had ever had one...

From the factory I have had two 48s, a 53 and a 51 with extra cylinders and none had a yoke.

I have had my Smith smith fix extra cylinders to:
35 (.22 Magnum)
63 (.22 Magnum)
48 (2) (.22 LR)
18-3 (.22 Magnum)
60-10 (.38 Supr/9mm)
649 (.38 Super/9mm)
66-2 (.38 Super/9mm)
17-4 (.22 Magnum)
51

And all just used the yoke that was on the gun...

So all you need is the correct vintage 17 or 18 cylinder which come up often on ebay and GunBroker...I have purchased all of mine there. Sometimes you get lucky and GunPartsCorp will have one but not very often...

Bob