I'm not with my RLs now, but wasn't it so that the threads for the screw are in the speaker itself and not the stand? If so, remove the support prop and the rectangular metal bracket under it from the speaker (only four screws) and take the bracket with you to a hardware store to try what fits.
It'll be a metric thread anyway... if you have a shop that sells screws, bolts and nuts by the kg, you could just as well get some of everything M4 / M5 / M6. One of them will fit, and it'll still cost pennies.