The Automatic Timing Unit.

The Instruction Book F.50 /4R refers to the B.T.H. unit type Y5 on Page 99. This unit is now superseded by the B.T.H. unit type YE5. They are interchangeable.

Later type units which do not have the limiting stop peg in the gear do not require the use of a sprag when being removed - see Page 46 of Instruction Book F.50 /4R.