On the UCSC ftp download site, there seem to be multiple options for downloading assembly data.
For example, the hg19 assembly can be downloaded from here:
ftp://hgdownload.cse.ucsc.edu/goldenPath/currentGenomes/Homo_sapiens/bigZips/
Or here:
ftp://hgdownload.cse.ucsc.edu/goldenPath/hg19/bigZips/
Judging by the file sizes and dates, these seem to be the same files, possibly even a symlinked directory.
Is the correct place to start for assemblies the ftp://hgdownload.cse.ucsc.edu/goldenPath/currentGenomes directory?