Yea, this is common in other areas
It's called contract manufacturing, and it happens all over the place.
You might own a plant for making something that has a capacity of X, but your entire market share for that product is something less than X, call it Y. You can just let your machines sit idle, which is stupid, or you can sell the excess capacity (X-Y) to your competitors.
It makes sense to, because after the introduction of the safety razor, there was probably little incentive for most Soligen manufacturers to invest in new equipment or facilities, which means that the few manufacturers who did keep up production would be in a position to sell the excess capacity to others.
Which is how you get lots and lots of razors that look like they are stamped out by the same machinery but have two different names on the tang.