The S2001A was based on the Sylvania 6146B with improvements. The 6146B was used extensively in radio transmitters especial for the Radio Amateur market. In the 1960s the Japanese made rigs were very popular worldwide and so it was no surprise that a home-grown output tube should appear.
A very interesting article by James Liles (K9AXN) can be found here and is reproduced by kind permission.
The two part anode is blackened on the outer face but has copper on the inner face.
The anode side flange with one plate bent outwards.
The wide glass tube envelope is 38 mm in diameter and excluding the IO base pins, the valve is 80 mm.
