The lockring dimension is the same as the M737 hub. The M738, as Charlieboy has already stated, was designed for use with suspension forks. It has a wider diameter, therefore stiffer, aluminium axle designed to reduce the lateral flex that earlier forks have.
It relies on doing the QR up really tight.
There's also a gold label LX M564 hub with the same axle.
Both were available between 1994 and 1997. Both phased out in 1998.
It's a bit like a precursor to modern thru axle designs, only less effective.
You can of course still M738 and M564 hubs with a rigid fork.