Be sure you're reading the manual for the correct version of the meter prism. You have a Photomic T (large aperture number slot in front, no release lever on side, no N on top). On the Photomic FTn, the later version with the small aperture number slot in front and big release lever on photographer's right, there are no silver nubs on the ASA dial, and you do lift up the silver ring and turn it to set the ASA.
This also matters because the procedure for indexing the lens maximum aperture to the meter prism is slightly different between the different versions of the meter prism, IIRC.