I wish I could provide you with the info you need. If no Twenty owners chime in with the precise information you need, I offer this.
Your missing bolt is likely either 5/16 or 3/8". Both of these sizes come in two thread pitches. Determine the length you need, make one trip and buy four bolts - one each of the following:
5/16-18
5/16-24
3/8-16
3/8-24
Perhaps if you explain your situation, the shopkeeper will let you return what you don't use.
Good Luck,
— Spalding
__________________
I came to say I must be folding . . .
Dahon Jetstream XP
Dahon Helios SL
Strida 5.0
Twenty project
— or not . . .
Fisher Mt. Tam (c.1988)
Merlin Road flat bar project
Schwinn Twinn Deluxe