During WWII a large quantity of quartz crystals were manufactured for use in military vehicle radios, and many of these ended up in the surplus market. There are two series of these ft-241 crystals, those marked with "two digit" channel numbers, and those with "three digit" channel numbers. These crystals are marked for frequencies in the 20 - 40 mhz range, but the fundamental frequencies are much lower.
Some of the crystals fall in the 455 khz range, and were used by homebrew radio builders in the 1950's to make crystal filters, especially for SSB exciters (see early 50's ARRL handbooks).
The history of these crystals is told with a lot of detail in the following site: http://www.ofc.com/library/vbottom.html .
Some of the following was described in the QRP-L archive, by Brad KG6IOE. http://qrp.kd4ab.org/2002/020320/0046.html .
These are currently (Sept 2003) available in full sets of 80 from Fair Radio Sales .
The actual fundamental frequency is the marked frequency divided by 54.
The fundamental frequency is also equal to: (20 + Channel/10) / 54
Channel Range: 0 ... 79 Marked Frequency: 20 MC ... 28 MC Crystal Fundamental: 370 khz ... 516 khz (about 1.8 khz steps in fundamental)
Example:
Channel 46 Marked Frequency: 24.6 MC Fundamental Frequency: 455.55 khz
The range of these crystals is 28 - 38 mhz.
The marked frequency in mhz is the channel number divided by 10.
The actual fundamental frequency is the marked frequency divided by 72 (or the channel number divided by 720).
Channel Range: 270 ... 389 Marked Frequency: 27.0 MC ... 38.9 MC Fundamental Frequency: 375 khz ... 540.27 khz
Example:
Channel 327 Marked Frequency: 32.7 MC Fundamental Frequency: 455 khz