This Sylvania valve was made to a US Government procurement contract and the Type designation VT-107B appears on the base cap. However, the valve is also marked as a 6V6G on the bulb.
The 6V6, 6V6G and 6V6GT family was, and remains, a very successful audio beam tetrode design.
The dome shows to interference fit top mica and electrodes. The cathode is rectangular, the control grid has copper supports, and the beam plates are clearly distinguished from a third grid.
The classic envelope is 44 mm in diameter and, excluding the IO base pins, is 103 mm tall.
References: Data-sheet, 1040 & 1043. Type VT-107B was first introduced in 1936. See also 1936 adverts.