Thanks for everybody who has responded, especially Dick and Tim who
sent me photos and a W&B sheet. I wasn't able to actually find the
dimples that mark the level plane on either side of the fuselage so I
ended up using the info in the BGA data sheet and leveled the aft
fuselage to 1000:51 downwards (1000mm:51mm level - and yeah, it was
exactly 5.1 mm down as in the diameter of a twig ;-) using the tip of
the nose cone as the reference datum (to hell with the MAC, wherever
it is supposed to be...) and the data for the arms contained in the US
Type Certificate (which specifies the arm for front & back pilot as
well as for the baggage compartment).

Turns out that our min single pilot weight went up from the original
68 kg to now 72 kg to stay within the CG range, probably largely to
the sturdy tail wheel with aluminum fairing the club had mounted on
the tail a long time ago in lieu of the joke that came from the
factory.

Thanks again everybody for helping to get this sorted out quickly.

Markus