Abas Major disassembly tips
John Griffiths, author of the Encyclopedia of Spring Air Pistols, offered this advice:
1. You start by removing the two grub screws in the muzzle cap. This enables the cocking lever pivot pin to be tapped out and the front sight to be unscrewed.
2. With these out of the way, the muzzle cap can be unscrewed. The gun is then straightforward to strip in the usual way.Two cautions:
1. The muzzle cap has quite a fine thread, so you have to be extra cautious that you don't cross it when replacing it under the pressure of the spring.
2. When removing the loading tap, be aware that there is a spring loaded steel ball underneath the tap lever, which can fly out and get lost if you are not expecting it.
He adds:
The muzzle cap may be be very stiff to remove because it has to be screwed in until all the relevant holes line up exactly, and some force may have been used previously to achieve this. Once started though it should unscrew easily.
I would advise you rehearse putting the cap back on a few times, to make sure there are no thread alignment problems, and also make a temporary marking to show where the threads first engage. The Abas had quite a preload on the spring, which made putting the cap back on a bit tricky, but [if] you are going to use a non-original spring I would go for a shorter one with little or no preolad, as this will make assembly and cocking easier. The small loss in power is neither here nor there for a vintage collectable like this.
Steve (in the UK) adds:

The front cap unscrews in a similar way to the front cap of Webley pistols like the Senior.
Looking at one of my old pics from when I refurbished mine, I can clearly see the teeth of the thread.
I recall I used the cocking lever (as I do with Webleys) to unscrew it PROVIDED it turns easily -which mine did.I recall that to remove the cocking lever I had to remove the seer as there is not quite room to take out the small slide that pushes the piston when you cock the pistol.
Importantly, unlike the Webley, whatever you use to turn the cap must not foul the threaded part of the barrel - ie it needs to be located close to the centre of the cap.