Just spotted this – how did you get on? I have a B34 and had to replace mine recently. I don’t know if yours are the same but my replacements are actually too tall to fit in the normal places (nav seat, cockpit lockers). It’s only a matter of a cm or so but is enough. I appreciate Bob’s comment about the pressure gauge but, unless you can find any the same size or smaller then I’d try to get the originals refilled at a reputable place. I have looked into every nook and cranny for an appropriate fixing point for the new ones but there just ain’t anywhere suitable. 5 years should be the minimum longevity for any extinguisher supplied in UK.