Well I've been looking at TCB clutches, the OEM (Aisin) clutch has two part numbers - one for early 185 and before and one for late 185 and after. These have slightly different (1mm) diameter spline specs. Heres the info:
165 and Early 185:
Part No : CTY41076
* Description : Clutch Kit
* Disc Diameter: 236
* Spline: 21x29.8
Late 185 and 205:
Part No : CTY41122
* Description : Clutch Kit
* Disc Diameter: 236
* Spline: 21x28.8
Since Aisin are the sole providors of gearboxes and clutches to Toyota (according to a long-term Ex-toyota man I know) I would suspect they are correct in their specification. So now it seems all aftermarket clutches are made to to early specs (29.8mm) in order to cover both but with the added risk of having a smaller amount of tooth contact on the late spec box. Obviously this saves on cost but will add problems with clutch alignment, off-centre loading of the clutch and early wear. Which explains why a lot of cheaper-end clutches (XTD etc) are prone to fail on this engine/box combo but seem to be fine in other makes of car.
I've measured my 205 box and that has a shaft spline diam of 28.3mm (thought it was 28.5 but its less), now assuming you have some assembly tolerance that leaves 1.6mm diameter difference between the actual 205 box and the aftermarket clutch (which is quoted as 29.8 by Exedy and a few others). Thats a HUGE amount of play IMO, far more than Ive seen on any other splined situation. I'll have a look at how my 185 clutch failed and see if its aftermarket etc... Wait for more info!