My guess is because it’s far easier to produce a new version of an existing instrument, in this case an A.S.I. Mk. IXF, rated to 480 m.p.h., than it is to produce an entirely new airframe and the engine that will power it. Hence, the lead-in times are rather different.
The Hawker Tornado was flying in 1939 with a Vulture engine. The original spec. called for a top speed of 400 m.p.h.
Your instrument may have been produced with this aircraft in mind.
In fact, and having checked my cockpit photo collection, I see that the proto Typhoon also had a Mk. IXF installed, and that was flying in early 1940, so it’s not really surprising that your ASI is dated 1939.
Totally agreed with that, and not to forget that this gauge (even if some could have been equiped with 0-400 MPH gauge) was also used on Beaufighters (first, were handed over to the RAF on 27 july 1940) Whirlwind (handed over on july 1940), Mosquito.
My own research on the subject shows that pilot’s notes are not the ultimate proof about instruments use, the operational use could be different.
Cheers,
Olivier
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