5 links
Bliley Electric Company History: The Early Years, 1930-1955
Bliley.net
History of the company that invented and patented the overtone crystal, etch-to-frequency and variable crystals. Source for ads, engineering bulletins, patents and manuals.
Crystal Info
pages.cthome.net
A handy historical chart of old crystal holders with dimensions, and suppliers.
Crystal multiplier chart
muchstuff.com
WB6SZS, Eugene Rippen's comprehensive chart for crystal use on the HF, VHF and UHF bands.