[Avodah] two fictional sects

[David Riceman]

RMB:

<<But Benei haBayshan are following minhag avos -- prior generations of 
Benei haBayshan. The moving of a minhag hamaqom when many of the 
residents of that maqom move is an instance of minhag avos -- the 
leaving generation doing what their parents did. It's not what their new 
maqom does -- it had no minhag. The only difference is that the BhB 
moved en masse. Kind of like the group relocation of some Chassidishe 
groups or of Frankfurt's KAJ.>>

Aren't you assuming your conclusion? The mishna talks about minhag 
hamakom.  The earliest source I can recall off hand for "minhag avos" is 
the Ran about Herem d'Rabbeinu Gershom.  What justifies your 
retrojecting the concept into Hazal?

It is plausible that a town adopts a custom in response to local 
circumstance: e.g., Tosafos say they marry their children off at a very 
young  age because they're afraid the local ruler will confiscate the 
marriage fee on another pretext if they save it until the kids are 
samuch l'pirkan.  They didn't retain the custom of child marriage when 
they moved to Poland.   In this case Rashi says the market day was now 
on Friday - - so plausibly the question was precisely whether a change 
of circumstance in the absence of a change of locality is sufficient.  
When the gemara associates minhag with location it may be implying that 
moving to a new location, even with the same population, provides the 
opportunity to revisit old customs.

David Riceman