There were likely significantly more chamfered mavic BB used on road bikes at the time, the late 80's starfish crank was supposed to be used with them if you went with an all-mavic grouppo.
I dont know that Ive ever seen an actual Mavic offroad crank in use. Even if you did not have the correct mavic chamfer cutting tool, for a quick one-off installation it would be a fairly easy task for most any machine shop with a vertical mill to cut the BB shell chamfers onto an existing frame.

Previous BF thread with Mavic BB discussion;
Let's talk about the Mavic 610/616 bottom bracket