I don't think any Mitsu trans from this era has a reverse syncro; at least not from the factory. I could be wrong, but it's like every Honda of the era and all others before that.
I have some pictures for you taken from the Mitsubishi service manuals.
Link provided to the manual for you to have at'er, and or for reference sake.http://www.lilevo.com/mirage/Manuals/FWD%20Manual%20Transmission%20F4M21-F5M2X-F5M3X-W5M3X%20Series%20PWEE8902-ABCD%2022A.pdf
Nearly all the manual transmissions repair manuals are located there. I uploaded all I could find from around the globe. This will help more than just our OP.
Spring update on that one.
I did post a pic of the F5M33 FWD trans end case, so you could see what that one looks like. The F5M31 is listed in the manual whenever you get to pics like above, it just doesn't list it in the titling of the manual and chapters, as such.
If you don't find what you are looking for there, it likely doesn't exist. I doubt that your FWD transmission will be any different than the ones printed in this 1989 copy of the factory repair manual.