My stepfather-in-law was kind enough to divulge some names and a little additional information regarding his family, which let me go digging around in the Lower East. I have candidate families for his mother and his father; further research will be required to make certain I have the right people. The truly bizarre bit that turned up was a _name_ coincidence that made me chuckle: the attorney who did M.G.'s naturalization in 1904 (hypothetical grandfather of stepfather-in-law) had the same name as my paternal grandfather after he came to the US a few years after that. At the time, of course, that man was a boy still in Fryslan, and still using his given name. Still, really quite hilarious to me.
This is the first time my genealogical efforts have revolved around careful analysis of street addresses on census pages. R. has been doing this for a while to disambiguate his extended family in Berlin, NH. The census form doesn't have a spot for a street address, but the name of the street is often written at right angles in the left margin or some similar spot. I had from e-mail the street where stepfather-in-law's parents lived (where he grew up -- not helpful just yet, because they got married in 1930, so that address won't show up until the 1940 census which I'll get my mitts on next year, IIRC), and also from where his grandparents lived. The candidate families both lived within a block or two, which I believed, based on what little I know about New York in the first quarter-ish of the 20th century, was important.
Having gotten this far, I took a big step back and contemplated the setting. What _did_ I know about the lower East? Hmmmm. I seemed to recall an unread book in my library about this migration and settlement pattern; I thought it was called _The Promised Land_. But I did not know if it had survived various attempts to reduce the number of books I keep around, and librarything was not encouraging. As luck would have it, however, a five minute visit to the third floor provided the book itself: Moses Rischin's _The Promised City: New York's Jews, 1870-1914_. Really, I don't think it could be more perfect for my purposes. Even having been written when it was (published originally in 1962) is fantastic, because Rischin was able to interview people who remembered How It Was Back in the Day before they all died or moved away.
Oh, wait, I almost forgot. A little surfing through the index for variations on a source-region theme (Ukraine, Odessa, etc.) turned up a map of the Lower East, with different lines drawn around various groups of blocks indicating "sub ethnic" groupings. Fantastic! Altho I then had to go look up "Galician" and similar. But so far, everything is looking just great.
ETA: Why, you might ask, did I own this book? I'm not entirely certain. It has a Half Price Books sticker on it with a 4/97, which looks like a date to me (that wasn't the price; that was $5.98). There was a period in the mid- to late 1990s when I was somewhat indiscriminately buying any used academic press book I ran across that looked interesting to me. This was particularly true when I was selling books and had a choice between more credit or less cash. I'd usually browse pretty aggressively while they were pricing the books I had brought in. If I found enough, I'd take the credit.
It's a little odd as a choice, even for my wide-ranging tastes. Yes, it is history, but it's not about food, textiles, technology or women, nor is it about religion. Fortunate, however. :-)