it's given as a variant in M-W Seventh Collegiate (1967), and as informal in AHD4; but I haven't seen a date for its origin.

edit: and here's the listing from the online 1913 Webster's:
Thru (?), prep., adv. & a. Through. [Ref. spelling.]

(the "?" is in the origin position; Ref. = reformed??)

Last edited by tsuwm; 03/07/2007 4:04 PM.