Erg make inverters, heres there info, if you ask they might tell you what it is.
Endicott Research Group (ERG), Inc.,
2601 Wayne St., Endicott, NY 13760, USA.
Tel: 607-754-9187; (toll-free: 800-215-5866)
Fax: 607-754-9255
E-mail: sales@ergpower.com www.ergpower.com
A wild guess and with the application I's say it was a current transformer.