IIRC 731 was a heat treated tube set marketed briefly by Reynolds in the early nineties.

I can't find any description of the alloy (whether manganese molybdenum or chromoly) or indication whether or not it may be cold set. As Zorro says, 753 (which is a heat treated manganese molybdenum alloy) should not be cold set.

EDIT - I found scans of an article in the Nov/Dec 1993 Bicycle Guide that discusses 731 OS compared to Columbus Genius.

LINK

Based on the article, 731 OS is the same composition as 531 (manganese-molybdenum) and the stays are heat treated and extremely hard. Because of this, I would be reluctant to cold set the stays.