If you have a B2620HST and do not know this you should.

[marrmanu]

This would also apply to the B2920HST and B2320HST.
The Operators Manual P. 65 does not mention removing the drain plug in the clutch housing cover.
The Workshop Manual P. S-14 does indicate removing this plug to drain the transmission fluid.
This might be common knowledge, but just thought I would put it out there.