I asked Douiglas Turner, Washingoton bureau chief of the Buffalo Evening News and former editor of the late Buffalo Courier-Express about beef on weck and he offered the following:

 The term is Kummelweck, mit ein umlaut uber das "u."
It is a hard roll, with the top gently split before baking, and with
generous amounts of heavy salt and dried dill leaves baked in.
These rolls can be purchased by the dozen at Kaufman's Bakery, 2381
Fillmore Ave., Buffalo, (716) 834 1800.
The only legitimate way to serve Buffalo's famous "beef on weck" is with
the meat sliced on site, and the rolls
  dipped into the pan gravy of the roast.
The sandwich is served today at Ulrich's tavern, Monday through Friday,
11:30 ayem to 3 p.m. Ulrich's is probably
the oldest tavern in Buffalo (city) in continuous operation, established
1868. It is at 674 Ellicott St., at Virginia St., and 
serves the finest German cooking found anywhere. 
[Ellicott, by the way, is for Joseph Ellicott  one of the surveyors who
worked for L'Enfant in Washington before
venturing into Western New York and Buffalo, as an employe of the Holland
Land Company.]
The sandwich used to be served in Buffalo at Meyers, at Wurzburger Hof, and
at Max'ls Brau Sturb'l.
Obviously, with Kaufman's turning out bagsfull of Kummelweck rolls daily,
it has to be served at plenty 
of other places. 
One further authority on this is Stephen Powell of Dorchester Rd., Buffalo,
who has written a history of taverns
  and Buffalo breweries called "Rushing the Growler."
 
Doug Turner, Buffalo News.