Yep, that's the "Hour Glass" box, so named because the worm is a complex machined piece that keeps the backlash constant throughout the travel and is shaped like an egg-timer. Fitted from October 1936 at chassis 257866 until the end of production (256501 for export models).
The ones I've seen break were all cracked and broken at the thinnest part of the arm, immediately behind the boss, carrying the ball joint connection to the draglink. Clean the paint off and have a good look, awkward I know. May be worth investing in a penetrant dye crack detection kit (3 aerosols).
Here in fact:
Ruby SBox.jpg (Size: 136.13 KB / Downloads: 145)
The ones I've seen break were all cracked and broken at the thinnest part of the arm, immediately behind the boss, carrying the ball joint connection to the draglink. Clean the paint off and have a good look, awkward I know. May be worth investing in a penetrant dye crack detection kit (3 aerosols).
Here in fact:
Ruby SBox.jpg (Size: 136.13 KB / Downloads: 145)