The Einstein Observatory (also known as HEAO-2 or HEAO-B) was operational from 1978 November 13 until 1981 April 25 when the Einstein attitude control system failed. The Monitor Proportional Counter (MPC) was a non-focal plane instrument that monitored the 1 to 20 keV X-ray flux of the source(s) being observed simultaneously by the operational focal plane instrument. The MPC was a collimated proportional counter filled with argon and carbon dioxide. It had a circular field of view, 1.5 degrees in diameter (FWHM), which was co-aligned with the telescope. A 1.5 mil Be window sealed the gas in the detector and shielded the detector from ultraviolet radiation. The active area of the MPC was 667 square cm. The MPC obtained useful data from 1978 November 19 until 1981 April 17, with the exception of a 3-month period from 1980 Aug 27 to 1980 Dec 08 when operation was intermittent, due to Einstein Observatory attitude control problems. The characteristics of the MPC have been described in detail by Gaillardetz et al.(1979) and by Grindlay et al. (1980). One duplicate entry was removed from the HEASARC implementation of this catalog in June 2019. This is a service provided by NASA HEASARC .