The 2901 is a spring return type.
The 1901 is motor driven to position, then the motor is powered off.

Sunvic say the two are wiring identical, but the actual valve differs
in some way. Any idea in what way please?

I'm getting fed up of the 2901 style of actuators suffering failures
and thinking to charge.

Harry Bloomfield pretended :
I'm getting fed up of the 2901 style of actuators suffering failures and
thinking to charge.

I did of course mean 2701 style...

A bit of in depth research on the Sunvic site suggests, that the
positioning of the D on the valve is not the same between the two
actuator models, for all three valve positions. Swap the model and the
valve also has to be swapped to match it. The 1901 seems to do a full
360 deg rotation to find its correct position, the 2701 only around a
150 deg partial rotation.

The thing now is - Is the later 1901 any more reliable than the 2701?
The 1901 seems to have design and component spec. issues, but they can
be more easily DIY fixed/ upgraded. I am also fed up of repairing the
spring return 2701 versions.