MXR Innovations was the largest US effects company, started in the early 1970s with Keith Barr as President and co-founder.
They closed in 1984, unable to remain profitable against Japanese brands (Boss etc).
In 1987 the MXR brand was purchased by Jim Dunlop, who started making reissues of the old line of effects and bringing new effects under the MXR brand to the market. [Source: P.A.S. ]
watch out for fakes http://www.analogman.com/fakemxr.htm