I was thinking of this frame:

but clearly there are many details that differ from the "Schroder".
Still, a custom building shop may have done any number of variations.
Adding to the mystery, here's a quote from the guy who put his name on that frame (from the CR list):
Lou,

I've also understood Schroeder frames were from Denmark. Here in SoCal
back in the 60's and 70's Henry Carstensen from Pops Bicycle Shop sold
them in Santa Ana, CA. Henry is Danish. He brought them in because they
were Danish frames.

Perhaps there is more than one brand with that name; possibly with
slightly different spelling or something?

Brian Baylis
La Mesa, CA
this was over some dispute as to the spelling (Schroder or Schroeder) and the origination (Danish from Copenhagen or Lund, vs German from Bremen)