20 x 1.5 mm sounds right for ER16, but that seems not be what the OP has.
ER collets are 16 degree total angle (8 degrees offset for the Topslide when boring a holder, ) for the longer taper, Can't remember what the angle for the smaller taper at the front, but 90 degrees total?
If you have thread gauges, measure the pitch, and then measure the diameter. Don't know the Toyo, but suspect that the thread would probably be metric.
possibly, you can find the angle for the internal taper with a vernier protractor. Or if you have a guess, you may be able to turn up bung, and check with blue. May very well need some trial and error and time.
Once you have thread and angle, you are part way to finding collets, but you need the right clamp nut to match the collets, so that they release when the nut is slackened.
HTH
Howard