The first type of Air Cleaners seen on Norton motorcycles were the louvred biscuit tin type circular Air Cleaners applied to the right hand rear side of WD16H motorcycles destined for Palestine and India in 1936. The rear wheel is valanced with a removable piece of sheet metal. The valance obviously serves as a shield to deflect stones and excessive dirt in general from reaching the filter.  This filter is described in 1936 spare parts list (contract C8753) with Norton type part numbers. The Cleaner was described in the Norton Assembly books as "Vokes C1a Protectomotor Air Cleaner", so in spite of the Norton part numbers, it was obviously made by Vokes.  Protectomotor was a brand name from Vokes describing a star shaped filter element in a louvred container.
Mechanisation Experimental Establishment  (MEE) tested the air cleaners on Norton WD16H (C 352877), Triumph model N (C 19467), BSA (C 33363) and Matchless 36/G3 (C 353555) in 1936.
The Norton, Triumph and Matchless had the filter mounted to the right hand rear while on the BSA the filter was mounted to the left hand front frame tube with the connecting tube attached at the bottom. A later type BSA was seen fitted with this filter in approximately the same right hand rear position as the other brands but the filter was rotated to fit the connecting tube at front.
 

The MEE considered the Vokes C1a Protectomotor adequate for its purpose but "making it more or less impossible to carry a pillion rider".  To reposition the filter, Norton designed and manufactured (still in 1936) a panel type air cleaner consisting of three felt tubes folded over a spring loaded rod at front,  having elliptical tubes to the header to which the felt tubes were clamped. "The element is protected by perforated metal covers and the whole cleaner was of tapered shape and a curved base to suit the top surface of a petrol tank of a 16H Norton motor cycle".
The MEE tests indicated that this configuration could be improved to reduce the dust gathering between the sides and the tubes which resulted in some erratic running and to increase the number of perforations in the sides and some modifications to the felt tube mounting and tensioning rod.

Based on these comments, Norton designed and manufactured a "modified" panel type air cleaner which in the end became the "Universal Air Cleaner", VAOS part MT12/NC/11783. (Within the VAOS spareparts system, part numbers with "NC" are Norton spare part numbers.)
Early 1937 MEE tested this modified version and found it "satisfactory for use" .

Between 1937 and 1942 this panel type air cleaner was manufactured by Norton and used on various WD16H versions for e.g. India contract machines and Australian Army  machines (See C7120 pictured above). It was also applied to the prototype Norton Big 4 (C 385695) of 1938.

In 1939 MEE tested the modified filter on Triumph 342cc (3SW, C38647) and on Matchless 346cc (G3, C3811084) to "ascertain the most suitable position on these types".  On the Matchless, the panel type was mounted on its side in the position as the previous C1a Protectomotor including a valance on the mudguard. Fairly peculiar in relation to the initial reason to develop the panel type filter in 1936!  On the Triumph, the filter was mounted on the petrol tank.

According to the contract ledgers, Norton delivered these panel type air cleaners upto September 1942 at which time a Norton order was cancelled and replaced at the Vokes company. Vokes was a renowned air filter manufacturer and possibly more suited for the manufacture of the air cleaners. What is not clear is that Vokes may have been the manufacturer of serial Aircleaners after the initial Norton development and that they were purchased by Norton and initially sold as a Norton product.

 

Filter identification by Norton, notice the non-adjustable straps fixing the filter to the kneegrip bolts and the fixed elbow to fit hose to carburetter.
 

(clik to enlarge)

A less well known Air Cleaner was the "Talflow Air Cleaner", a product made by the "the Talboys Manufacturing Co. Ltd.". 
Talboys was co-founded by W. Mansell (director of Norton Motors) and made sheet metal products for Nortons, petrol tanks for certain and oil tanks most likely.
The brass plate found on a number of filters shows a "patent pending" text. This patent was initiated in 30th July 1940 and finally accepted on 7th November 1941.
A fair number of cleaners surfaced in 2020 without the brass identification plate.
This Air Cleaner has an internal rack with staggered screens which needed to be oiled in order to "catch" the dust particles.
The rack was easily removable from the casing for cleaning and oiling. The entire rack was also wrapped in a textile/felt material soaked in oil.
Rack and felt are to be cleaned and oiled with a frequency depending on the dust circumstances.

Contrary to expectation (the link with Norton director Mansell), only one picture has surfaced until now (2020) showing a Norton with this Talfow Air Cleaner.
One available Contract ledger entry shows an order for 13.472 air cleaners for "motor cycles India" around September 1943, indicating a far East link.
Norton engine books indicate that new WD16H machines delivered to India Office in 1946 were provided with Talflow Air Cleaners.
 

Some history on the "The Talboys Manufacturing Company Ltd." firm can be found on https://www.gracesguide.co.uk/Talboys_Manufacturing_Co
 

Brass identification plate