This is a vacume guage for 2 cyclinders.
For those of you who may have a twin cyclinder still lying around.
If this is any use this post could be moved to it's correct location by the crew.
http://www.airheads.org/index.php?Technical%20Tips+%244%20Carburetor%20Synchronizer
I found it while browsing the net and think it may be useful.