What is the origin of mule in test mule?

It may have originated in Italian rather than English, because the 1964 Road and Track vol. 16, page 43 says:

had built a pair of muletti — "mules" — whose design had been hastily roughed out by the same internal talent that had drawn up the Dischi Volanti and many other "house" designs. The workmanship of these muletti also was rough as they were never intended to be seen by the public.

There are slightly earlier examples in English:

1962 Road and Track, Volume 14, page 124:

After this crash, the practice/testing "mule" was hauled out, the salvageable pieces from Gurney's previous mount installed. On the day following the crash, Dashing Dan qualified at 149.019-mph, what was ultimately to be 17th fastest.

1960 Car & Driver, Volume 6, page 46:

With a rough fiberglass body this became the "Mule", which went down to Sebring for on-the-spot trials while the actual race car was completed... the Mule was revised and cleaned up in detail to be exactly like the race SS, but the ax fell on the project before the ex-Mule could be assembled ... this car, the Mule...

1957 Car and Drive, volume 3, page 72

...while the SS in both "Mule" and "show" variants ran they went like stink. The officially released lap time set by Fangio at Sebring in the prototype Mule was 3:27.2, a very respectable figure.

1956 Road & Track, volume 8, page 220

Chevrolet's practice car had fiberglass body, was called "the mule"

There is earlier literal use of "test mule", such as in the 1955 Journal of the American Veterinary Medical Association, Volume 126, page 285

When test mule 1434 and test horse 1121 were subsequently challenged with a known strain of virus, both promptly developed clinical symptoms of infectious anemia

In French or Italian, car racing teams were/are called "écurie (de course)" or "scuderia", literally racing stable: the race cars are the horses and the replacement car is called in French "le mulet" (the mule) and in Italian "il muletto".

Both in French and in English, the sense of mule/mulet later extended to development cars (testbed vehicle equipped with prototype components requiring evaluation).

The italian Wikipedia article on muletto refers to a French origin of that Italian word, because in italian, the animal is a "mulo".

However, I have any other evidence to support a French/Italian origin of "mule" in the sense of evaluation prototype. Even if the word is quite common in French, I only found one occurence in the very view Google books on car racing: the book was written in 1958.