Every German Welte-Mignon I've ever seen has these grooves in the
tracker bar -- my 1923 'T98' certainly does, exactly as per the photo
posted in MMD 170213 (which is indeed a T98).  This is exactly the
design initially shown in the post that triggered this thread, "Extra
Holes in Ampico B Tracker Bar".

So, it's an Ampico fitted with a Welte 'Green' T98 tracker bar --
the only Welte model that would fit onto an Ampico, because the T98
is the only Welte that uses the standard 9/inch scale as its basis.
That was to make these machines compatible with standard 88-note
rolls, including having the sustaining and half-blow pedal ports in
the same place as Aeolian rolls, and the expression ports placed so
that a Themodist "snakebite" triggers an accent in the Welte system.

The pedal-electric T98 can therefore play theme-accented rolls such as
Themodist and Hupfeld (it even switches the stack split position when
this is chosen).  Mind you, sales of this rather sophisticated model
can scarcely have justified the investment, as most of the already-scarce
T98 instruments are only setup for Welte rolls.

Julian Dyer