The company’s adverts of the late-20’s talk of the ‘Weller box’ and motoring publications of the time refer to the ‘Marles steering’ on AC’s. It may be that John Weller's brother designed the AC box as he patented gears used by AC in the mid-20's. His first patent on helical gears for Marles was not until the 1930's.
The vintage box on AC's is not one of Marles usual designs; it is similar, works on a worm and roller principle (though, unusually, the roller sits parallel to the worm, not at 90°) but the casing is different, there is no makers name on it, and it may be unique (so it is not quote 'a close copy' unquote). Maybe the build was subcontracted to Adamant Engineering (the manufacturers of Marles boxes) or it is very possible they were made, at least assembled, in-house. The owners’ booklet does describe it as ‘the AC patent steering’.
It is prone to leaking after so many years and much debate has centred on the use of modern semi-fluid grease to limit leakage. Do not use ordinary grease, the recommended oil was Price's Amber 'B' (now Castrol D140 is commonly used). There is only one seal inside and there are two adjustments possible. A worn worm could be a serious problem - not least getting acceptable tolerance when working with hardened steel with a newly-made one, let alone the cost. The easiest removal on the vintage AC - although it may not sound it - is through the floor by removing some of the wooden sections inside the cabin. Some boxes, like on Lagonda and Alvis are visible and easy to get to, unfortunately not so on the AC.
I have never heard of an AC suffering from cracked worm, and think it would be most unlikely considering in-house design and possible manufacture.
Postscript: should add, the above refers to the six-cylinder.