In the olden days before limited-slip differentials were widely economical and available for small roadrace cars, a locked rear gear was just about the only option. If you had one in your Sprite you learned to drive it and you beat the guy with an open differential. In the associations where locked rears were illegal, part of tech inspection was to push your car around in a small circle; the shudder of racing tires being dragged across pavement was a giveaway.
The economical way to lock the gears was to place the appropriate, assembled parts in some kind of bowl and pour enough molten lead in to accomplish the locking. That way you could easily recover the gears if you needed to. Something to think about in case someone is bent on trying a 'spool'.
I raced an MGB with a Detroit Locker. It took adopting some technique changes, but was indispensable once acclimated.
