Error in this depiction of a Stirling engine?

[greypilgrim]

Hi.
In my (German) physics lecture notes, I found this depiction of a Stirling engine:

According to the text, it should be
A: isothermal expansion
B: isochoric cooling
C: isothermal compression
D: isochoric heating

This seems correct for the PV diagram, but strange in the illustration on the right. Shouldn't A and C be (approximately) isochoric since that's where piston A (green) is at dead centre?

 

[mfb]

It looks like the labels are shifted by one phase. B in the PV diagram is A in the sketch and so on.