If it's like my JMW 10 - and it looks like it is in photos - there should be a set screw on the left side, just below the height adjust knob. You can adjust the arm to spindle distance by rotating the arm base. I used the JMW jig and snugged the base into the jigs "vee"
Only the original JMW 10 and 12 arms had a "dog leg" support that could be pivoted to adjust for S2P distance. 12.5 and 12.7 arms have a fixed support, therefore, you will need to adjust the entire base to achieve the proper S2P distance.