Hello! The Jag-Stang tremolo/bridge is basically just the Mustang tremolo/bridge assembly. As far as I knew they are all the same no matter if the jag-stang was 'Made in Japan' or the 'Made in Mexico' issue. That being the case I've always recommended Mustang parts when it comes to bridge/tremolo. Just be sure to order parts from a place that allows returns if they don't fit.
I have no idea why you're seeing a size difference in your measurements. It's my first time hearing of this. If you have a chance to compare directly with a mustang tremolo please share more info on size differences.
Since you're missing a Trem Arm you're probably also missing the "tightening screw" so start looking at this info as well. https://www.jag-stang.com/news/tightening-that-dynamic-vibrato-tremolo-arm/
You may end up being better off buying the entire tailpiece if you need the screw as well. This is just an example at warmoth: https://warmoth.com/mt1c
