Those who sign the abstinence pledge are entered with O. P. (old pledge) after their name. Those who pledge themselves to abstain wholly from alcoholic drinks have a T (total) after their name. Hence, T = total abstainer.
The tale about Dick Turner, a plasterer or fish-hawker at Preston, in Lancashire, who stammered forth, “Ill have nowt to do with the moderation botheration pledge; I'll be reet down t- total, that or nowt,” is not to be relied on.
It is said that Turner's tombstone contains this inscription: “Beneath this stone are deposited the remains of Richard Turner, author of the word Teetotal as applied to abstinence from all intoxicating liquors, who departed this life on the 27th day of October, 1846, aged 56 years.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Teetotal from Fact Monster: