Missing T

a.k.a. T Puzzle, Magic T, Cut-Up T, Pa's T Puzzle,
Great American T Puzzle, White Rose Ceylon Perfect T Puzzle, Lash Bitters T Puzzle.

Old design, copyright 1898 Lash Inc., this copyright ThinkFun Binary Arts 2003.
(plastic, 4 pieces, 3.1" high by 3" wide when solved)

The four pieces can be positioned to form a T as shown above. The HIQU puzzle has a booklet of problems based on these pieces. This is an old puzzle that has been produced many times; here is a nice antique wood one:

Pa's T Puzzle, circa 1940's.
(cardboard box 4.6 by 1.75 by 11/16 inches and 4 walnut pieces,
5" high by 4.75" wide when solved;
box edge says "PA'S T PUZZLE No. P 20 WM. F. DRUEKE & SONS Grand Rapide, Mich.")

Adams Version Of The T Puzzle With Solution

S. S. Adams Co., circa 1950's.
(3.5" cardboard sleeve with cardboard directions and dark green plastic pieces)

Example Of Inexpensive T Puzzles Sold Circa 1960's

Magic T Puzzle, unknown manufacture.
(plastic box 2.5 by 1.75 by 1/2 inches and 4 plastic pieces)

Marx Toys, circa 1960's.
(5.5 by 7 inch cardboard card with plastic pieces;
solution sheet in plastic bag and big question mark with company logo behind the bag)

Further Reading
T Puzzle Wikipedia Page, from: http://en.wikipedia.org/wiki/T_puzzle