All of its units are 'American', especially airspeed is in mph which seems very odd when the dial is in Knots.
That was a problem in around the "M's" of the Cessna 150 and 172, Cessna was transitioning from MPH to Knots, and I have noticed that some planes had a mismatch of speed units ASI to flight manual. In most cases, the Cessna ASI's had two scales, both MPH and knots, so it was more a matter of which was the outer scale. In any case, the coloured range markings will be correct, even if the speed numbers primarily scale is in the other units. Interestingly, Canada, being a "metric" country, still has aviation regulations stating that primary aviation units shall be imperial. Imported planes with Kmph ASI's and meters altimeters must have them changed to imperial units for Canadian registration. We had our teething troubles with the metric changeover - see "Gimli Glider". We've never done passenger weight in stones.
I'm not aware that the Tiger Moth was provided with a flight manual (I asked, and was told not). I was asked to fly the maintenance check flight for a Tiger Moth following a ten year rebuild (Apparently, the owner, who so eagerly awaited the opportunity to fly his pride and joy, following the rebuild declared that the cockpit had shrunk during the process, and he no longer fit. I do know him to have enjoyed a good meal from time to time). Anyway, I gave it a careful walk around, and strapped in. It had been 30 years since I'd flown a Moth, so, with nothing to read, I took my time learning around the modest cockpit. While doing a control check, I noticed that ailerons move up when they should, but the opposing aileron did not move down much. With no written reference, and no training for this. I unstrapped the airplane, went up to the owner's office, and asked. Yes, they worked correctly, so I flew it.
For aircraft for which the manufacture provides a flight manual, it is required by regulation to be accessible to the pilot in flight, and the pilot is required to fly in accordance with what is written there.
It is as important to be familiar with the contents of any flight manual supplements (FMS), as the flight manual itself. The flight manual supplements have the same importance, and regulator requirement as the basic flight manual. These days, older aircraft are being modified to newer standards, and that often affects aircraft systems (avionics/electrics in particular). I oversaw and approved a Cessna 182 conversion, which when completed included 27 STC mods to the aircraft. Many of these mods were accompanied with their own FMS, though some not. My job became writing yet another FMS which drew together all the relevant differences in the final aircraft. It is 18 pages long.
Every FMS will probably say something like "this is not a substitute for competent flight training", and I can go along with that - you still need to receive good flight training to know how to fly! But with that, the flight manual should bring you to the required familiarity with the aircraft type. When I was a lowly 150 hr PPL, and worked a junior office position at an airline, I also moonlighted for that airline as a sub assistant sim tech for the DC-8-63 training simulator the airline had. It was a full motion, with very limited night only visual display. My training was simply to shut the sim down when the crews were finished training at midnight or so. So I'd spend all evening in the sim office, doing whatever I wanted, simply being available to shut it down, if something went wrong. So I read the manuals - all of them - over and over. Then, I realized, that when the crews were done, I could lock myself in, and go sim flying. I taught myself to fly a DC-8-63 from the flight manual - zero dual instruction. Not wanting to get caught, I always had to leave it parked at the runway threshold where I'd found it, ready to go, and I did. I never crashed it. After 40 or so hours of this (which I informally logged for myself) The boss found out, and joked that maybe they could give me a type endorsement based on sim training only. Happily, I became the go to office guy to give visiting office guests a circuit in the sim over lunch. The greater message to me was that the flight manual, if well understood, contained the information needed for an adequately competent pilot to familiarize themselves with the aircraft enough to fly it with adequate safety. Many times since, I've checked myself out on a new type, from reading the flight manual, and the FMS'.