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Joystick Adaptor Plates
Not every joystick fits every panel. Most Japanese arcade panels used an adaptor plate, to which the joystick was affixed before being attached to the panel.
You can have a look at the panels and sticks on these pages:
SNK
Used on the Super Neo 29 Candy, and probably others, this mount is the same as the ones used on Sega's Astro City panels (which were also used on many Sega cabs) except the depth is 10mm, where Sega's is 14mm. It uses only two screws, at 95mm, to affix the plate.
Several sizes of plates will fit here. The plate shown is for a Sanwa mech, plates for Seimitsu sticks are larger than Sanwa plates (64mm wide). The two important dimensions are 96mm for the mounting screws, and 10mm depth. You should also make sure the central hole fits your stick, but anything over 21mm should do.