when you come right down to it, it really doesn't matter what some manual says.. That manual refers to a brand new bike in factory tune.. everything is new.
you got an old bike.. and yeah, someone may have dicked with it.. with more than just the mainjet.
You can determine what mainjet is best for that bike with the stuff it now has on there (exhaust, air filter, tuining condition etc. by doing a plug chop.
Buy a few brand new plugs. WArm the engine up. Drive over to a long straight flat road. Install the new plug, get to top speed for about 1/2 mile and kill the engine.. "chop" the engine's electrical while at top speed to kill it.
coast to a stop, remove the new plug, install the old plug and drive home.
Examine the new test plug under good light and magnification. If there is any carbon duston the plug the jet is probably too big. Carbon is unburned fuel. The insulator should come out clean and white like it went in (not grey .. overheated). The electrodes' edges should be sharp under magnification (not melted round)
I say probably because not enough air intake (clogged air filter) could falsely show the same thing. a rich mixture.. so will ignition timing that's way off.
So, tune the bike as best you can.. no air or fuel leaks, no nothin, before doing the plug chop. Get it in best tune possible. Use only a new plug and only when the bike is thoroughly warmed up .. and dont let the engine idle or run below top speed... chop it off.