As stated in the description of the two items in the Märklin product database, the mounting of the pickup shoe in 73404 is asymmetric and in 73405 is symmetric.

Regards
Tor

73406 has the clip mount at one end of the base plate. 73405 has the clip mount precisely in the centre of the base plate and 73404 has the clip mount slightly off-centre near the middle of the base plate. Each one is to be used with different models, depending on the design of the model. Some coaches have the bogie pivoted at the centre, some have the bogie pivoting slightly off centre and others have the bogie pivoting with the mount at the rear end of the bogie. The correct use of the slider means that for that particular model, the slider will line up with the bogie and won't extend too far in front or behind the bogie, which could result in the slider interfering with the coupling mechanism or with the underside of the coach body.

The decision to design the bogies with off-centre pivot points reduces the overhang of coaches in model railroad curves, as the off-centre pivot allows a 27 or 28 cm coach to have a similar overhang to a centre pivoting 24 or 25 cm model, so the models look better on R1 curves and do not interfere with signals or catenary masts during operation.

Regards

Mike C