I've been doing some research on The Massey Vice Co., of Chicago, IL, and not yet found a connection to Massey Harris, but your recent post makes me wonder if there are several M-H vise models out there.
Does your vice have a feature on the top surface of the traveling beam which provides a means for shearing rivets on a sickle bar mower section?
This was a feature patented by Howard Milton Johnston of Toronto (of Johnston Harvester fame?) and assigned to Massey-Harris Co. Ltd. of Toronto. Patent was filed on Dec. 19, 1936. The patent drawing of the vise and showing the feature is attached.