I have a Schwinn GTX3 Hardtail with a M3010 Suntour fork. I looked up the model number on the back of the left leg and it is supposed to have a preload adjuster available. However, mine does not for some reason. I was wondering if it would be possible to install a preload adjuster or lockout if I buy the parts, even if it does not have them on currently. (Btw: The fork is the one that originally came with the bike model.)
The following exploded diagram for something called SR Suntour SF15-M3010-A-P-26-50 lists a "CONTROL ADJUST SYSTEM" as part number FKE007-11 . Depending on whether your fork is a compatible one or not, this might be the part your are looking after.