I am reasonably new to old bikes, but here is what I do with my bike; hopefully you might be able to compare things to yours:
The MO1L magneto on my Panther has the advance retard cable entering from the right hand side, when viewed from the points. When I turn the engine over, the points rotate clockwise. When I tighten the cable, it rotates the face cam anticlockwise (i.e. opposite direction to the points rotation), causing the points to open earlier than when the wire is slack; thus it a tight wire advance magneto.
With the Panther, and I imagine all bikes with these magnetos, there are two ways to set the timing (obviously, I set the points gap, of 12 thou, beforehand) -
1/ With the cable tight, so the lever is in the full advance position, have the points just opening at a specified angle before tdc (for my Panther it is 33 degrees), or when the piston is a specified distance before tdc (11 mm for my Panther).
2/ With the cable loose, so the timing is fully retarded, have the points just opening when the piston is at tdc. This is easier, but it does allow the possibility of over-advancing the timing, which can have disastrous consequences.
I use the 1st method.
I hope this might be of use to you; obviously you'd need to determine the appropriate degrees or distance before tdc for your bike. BSA Service sheet #604 says 7/16 ths btdc but 3/8 ths for models with engine prefix GB.
Sheet can be found here: https://www.bsa-oc.com/wp-content/uploads/bsk-pdf-manager/95_SECTIONM_BSASERVICESHEETNO604.PDF