Pitot heads
The 'calibrated leak' is common on lots of old British aircraft. Same effect on Scout/Wasp helicopters, Chipmunks etc. You can carry out a system leak check if you have the correct long head adaptor that also covers the static slots in the side of the head so you apply static to both pitot and static simultaneously. Don't apply pitot or you'll drive the altimeter backwards against it's stops and risk damage. The ASI will be fine because it will see static inside the capsule as well as outside so it will be balanced. There will be no reading on the ASI, just on the altimeter.
Hope this helps.