The only alloy with which I am familiar that has a similar designation is a magnesium alloy produced by Magnesium Elektron in the UK called MSR-B. I have attached a link to their web-based datasheet:
Quite often the MSRR spec. will reference the equivalent. I also believe (know!) that there is a RR spec which lists the equivalent international specs. for a great many of the MSRR material specs.
Do you know any RR Metallurgists.?? you might be lucky and find a benevolent employee who is willing to help you out.
I just did a little favor for an RR employee via the ASM forum so he will probably be willing to help. So give me a little more detail about what you are lookinkg for and I will pass it along.