The Mules used to be known as MD-3's, but are now officially known as the A/S32A-31. Quoting from NAVAIR (Naval Air Systems Command) 19-40-44:
"The MD-3 Tow Tractor is a pneumatic tired, diesel powered tractor intended for towing all carrier based aircraft aboard aircraft carriers. The MD-3 tractor is powered by a 100 horsepower diesel engine and has a draw bar capacity of 8500 pounds at an approximate speed of 1 mph on a dry level concrete surface. The MD-3 includes a quick detachable unit with the capability of starting conventional and impingement type air-starting aircraft and providing AC and DC electrical power to aircraft while to towing. Hydraulic power steering is provided to permit maneuvering in confined areas. The tractors have a low silhouette to permit operation under wings of parked aircraft."
"The tractors originally came into service in the early 1950's and were manufactured by Frank Hough Corp. Around 1952, the Frank G. Hough Company was bought out by International Harvester which was later bought out by Tenneco Incorporated and merged with its then subsidiary, Case Corporation. The tractors originally procured in the 50's were SLEPed (Service Life Extension Program) in 1981 and reidentified as the A/S32A-31. This SLEP was essentially a re-engining of the unit (new engine and transmission), along with some brake components and a general overhaul (stripped all the way down and repainted)."
It seems the original vehicle was retrospectively designated A/S32A-31 and or A/S32A-31A. The SLEPed version is most likely designated A/S32A-31B, but this is not confirmed yet. The B version can be identified easily by the widened rear of the vehicle (some 6 inches). Some Mules also have a sloped rear edge, so far I've only seen whites Mules with this 'spoiler'. It's also likely that the repainting mentioned above indicates the change from yellow to white. The Verlinden, Fujimi and Maquettes Dauzie Mules are the early narrow version, only DreamModel offers the later wider version. Note that some white (repainted) Mules don't have the widened rear side.
All kits I've seen have single rear wheels. But I think the real thing has double wheels. Possibly the 6 inch widening also has to do with the rear wheels.
Confusingly, the US Air Force also has an MD-3: "The MD-3 Generator Set is a portable unit designed to furnish AC and DC electrical power for the starting of aircraft, and for ground maintenance". It's definitely not a tractor, so the two can't be confused.
In 2000 I obtained a resin Verlinden 1/72 scale model of the MD-3 'Mule' US Navy deck tractor (catalog number 240) at an IPMS meeting. It had been on my wish list for a long time, but I never found one. Apart from some pinhole air bubbles I liked it a lot. The only thing I planned to improve were the wheels, since the tires lacked profile, which was a bit akward for such a slippery working place. I planned to use the wheels of a Fujimi Mule, that reportedly had more detailed wheels, but otherwise would be inferior, I thought.
Eventually I bought the Fujimi 'Deck crew set' (catalog number 35001), and this turned out to be quite a surprise. With regards to details, this injection moulded MD-3 was a near 100% copy of the Verlinden model! Of course Fujimi had to break up Verlinden's single-piece main casting into several smaller parts, but otherwise the differences were very, very small. The hood has a fold on the centerline (that I have seen in only one photo of the real thing, the rest had slightly bulged hoods), but otherwise it is identical, down to the tiniest details. I was quite amazed that it was possible for Fujimi to achieve the same detail level with injection moulding. Another lesson learned!
Since the Fujimi set is on the market for quite some time, it could explain why Verlinden possibly stopped production, and why I couldn't find a Verlinden Mule all those years. I haven't nailed down the exact vintage of both models, but the Verlinden Mule is probably from 1987 or earlier. It was produced until at least 1994, since it is listed in Verlinden catalog #13 1994-1995. I found the Fujimi set in a 1992 mail order catalog, but it could be older. In 2008 or 2009, the Mule tractor was included in a reissue of on Fujimi's VF-96 F-4J.
The Fujimi set was also issued by Testors in the USA, with catalog number 850, with photo box-art. Possibly it was also sold bagged, with a black and white header card based on the Fujimi box art.
In 2007 I bought a Maquettes Dauzie resin 1/72 scale US Navy tow tractor (catalog number 72 006) at the IPMS-NL Nationals. Compared to the Verlinden/Fujimi MD-3, it has a different unit attached to the rear side. Also included are parts to make the air starter's hose, in the shape of a piece of hose, and metal wire to wrap around it. The same tractor was also available without the rear unit, catalog number 72 007.
The length and width of the complete vehicle are around 15% less than the Verlinden/Fujimi Mule, which makes you wonder who used the correct (or best) data. I obtained some more MD models, and the dimensional errors in them are so obvious that my trust in MD's master builder has disappeared largely. Therefore I believe the Verlinden/Fujimi Mules are more accurate.
In 2008, DreamModel from China issued four resin Mule kits, with photo-etch details and decals. The versions are old & new short style, and old & new long style tractors. The new style tractors have increased rear track, and slightly widened fenders to match.
I haven't assembled the kits yet, but the Fujimi kit seems to hold the edge. Some advantages:
NAVAIR (Naval Air Systems Command) document 19-40-44s lists the following dimensions, without stating whether it's for the original design or the SLEPed version: 162" length, 65" width, 36" height, 11" bumper height, 4" bumper depth, 7.5" ground clearance.