The T stands for “Touch” and the early TW-1’s actually had the text Touch Wah written on them instead of T Wah. The TW-1 was replaced by the AW-2 in 1991. Together with the CS-1, the TW-1 is the only pedal to have a toggle switch. With only 2 controls, the TW-1 is very easy to control. The later autowah pedals have the added versatility of modulation but for the basic wah sound the TW-1 does the job without forcing the user to read the manual.

Version Differences
The earliest TW-1s was labelled Touch Wah instead of T Wah
There were both silver screw and black screw versions made
The T in TW-1 was first placed under the W in T Wah but later moved under the a

TW-1A
Serial: 7300 – 9000
Marking of PCB: 052-287A
Switch: Up to 8500 transparency, from 8600 black
Silver screw
OP-AMP: RC3403A
Momentary LED

TW-1B
Serial 9100-0300
Marking of PCB: 052-287B
Switch: Black
Silver screw
OP-AMP: RC3403A
Permanent LED

TW-1C
Serial 0400-
Marking of PCB: ET5206-510B
Switch: Black
Black screw
OP-AMP: 2x uPC4558C or JRC4558D
Permanent LED

Specifications
Controls: Sens, Peak, 2-way Drive switch
Connectors: Input, Output, AC Adaptor
Current Draw: 6 mA (DC 9V)
Recommended AC Adaptor: ACA Series

Labels
Black – Made In Japan
The TW-1 was sold from July 1978 to May 1987.

http://www.bossarea.com/boss-tw-1-t-wah/