Greetings all,  In my travels and inquiries, I have never come across
a T100 Welte system in any other American made upright piano from 19xx
to 1913, other than a Mason & Hamlin or a Krakauer.  Factory shipping
records suggest that Mason & Hamlin shipped some 200 uprights during
this period, the majority of them were straight pianos.  The cases were
extended in the front approximately 8 inches, to accommodate the T100
stack & the pump downstairs.

I know of eight Mason & Hamlin T100 pianos.  The serial number is
usually found under the keybed, stamped in one inch letters, all in the
22,000 series.  The letters & numbers stenciled on the plate designate
the player and the system.  The string plates say Mason & Hamlin, the
fallboard decals say Welte.  Some are "Art Case", others are plain case.
The castings on the plate often have the manufacturing date, usually
some month in the year 1913.

I feel sure some others can shed more light on the question, and
I offer this information with the anticipation of correction.

Happy holidays to all,

John McClelland

 [ M. Welte & Sons purchased the deep-case Mason & Hamlin and
 [ Krakauer pianos outright, apparently under a contract that allowed
 [ the product to be marked as though completely built by Welte.  See
 [ http://www.mmdigest.com/Gallery/Pictures/Welte/MHwelte.html 
 [ -- Robbie