Thanks Greg for the link to the
new location on Mamiya's site for these manuals.
Unfortunately, you won't find an instruction manual there for the finder you are using.
I use the predecessor to that finder (M645 Super AE prism finder N) on my M645 Supers and my M645 Pro. I believe it operates in a manner similar to your 401 finder.
Here is a link to the instruction manual for it:
http://butkus.org/chinon/mamiya/mamiya_m645_super_ae_prism_n/mamiya_m645_super_ae_prism_n.htm
With my finder, if I have the camera set for manual exposure, the manually set speed is illuminated, and the suggested speed flashes.
In the manual for the Pro-Tl there also seems to be a suggestion that if you forget to take out the darkslide, the shutter speed indicator flashes as well.
Hope this helps.
Matt