The R3 is not listed, its as if it did not exist. The R3 that is listed is a Full Wave Rectifier. This valve has to be a replacement as the envelope clearly shows that it is gettered. The electrodes are obscured but looking in from the base, a horizontal R style assembly can be seen.
The etched markings on the main picture show a triangle with an M at the top and the R3 below.
The picture above shows the pinch and the base pins.
The wide glass tube envelope is 30 mm in diameter, and excluding the B4 base pins, is 65 mm tall.
Updated January 17, 2013.