Yet another cool old 5-pin, the Duprat hollow steel cottered. Not made like the Wedge-Lock out of tubing; these were normal forged cranks that were drilled, from the BB spindle end, almost all the way out the pedal end. Then the BB spindle end was forged shut, except for that thin slit you see (info from their patent application). You can only see the hollowness by taking the cranks off the spindle and taking the pedal out, but I think any Duprat with the slit opposite the cotter pin hole is the hollow model.
Not only are they hollow, but also very slender on the outside too, compared to aluminum cranks. Pretty lightweight. Lots of testimonials saying they had decent strength/reliability, despite the lightness. Ted Ernst is a renowned racer (and all-around cool dude) who raced everything, but especially track, including 6-Days in Europe and in America. He had this to say:
"When we were selling Urago back in the ’40/50’s all the better models we ordered were with Duprat hollow cranks.
I raced on them for about 5 years, and never had any problems.
Great float feeling and easy to ride for hundreds of miles.
We never had one break on any of the bikes we sold, whether to racer, sport, or touring rider."
5 years under Ted Ernst is like a lifetime of riding by some mere mortal.
In case anyone doubts they're really hollow, here are some pics of a crank that some nutjob sawed in half the long way. It shows how they drilled it to two diameters with a step partway down.
They never were common exactly, and much rarer now, but still a decent number of them survive.
Mark B in Seattle