Having had a brief rummage on the internet, it looks as though the 'A' set aerial base was fitted to the bracket on the centre of the turret bustle (in place of the huge American insulator), so would use Base No.8, on Mounting No.3, and a Plate, Connector, No.2 and insulated wire to connect to the variometer. The 'B' set mount appears to be on the turret roof, to the left of the centre line and forward of the turret hatches, and is on a short mounting (Base No.9, Mounting No.1) with the Protector (ring) fitted. See:
http://www.tanks-encyclopedia.com/ww2/US/M3_stuart.php
and
http://www.weirdwwii.com/2012/03/bat...wealth_28.html
No top views of the turret, unfortunately.
Chris.