The Efratom division of Ball Corp., based in Irvine, CA, produced rubidium frequency standards and related instruments. Efratom was acquired by Datum, Inc. in 1994 .
Efratom made plugins for frequency measurement that were compatible with the TM500 system.
From this list:
-  Anaheim’s Datum Inc. to Acquire Efratom L.A. Times, 22 Oct 1994