This exhibit is branded as being made by Hammond on the base cap with the Type designation and country of origin etched into the glass.
We have data from Westinghouse who describe the 7591A as a power output tube that is capable of 45 Watts of output at very low distortion when operated in push-pull and Class AB1. It is pin compatible with the 7591.
The working face of the anode has the extra heat sinking fins. The bright beam plate is just visible inside the anode.
Through the holes in the anode can be seen the beam plates and both wire grids.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 70 mm tall.
Reference: Data-sheet. Type 7591A was first introduced in 1960.