Spindle collet cartridge mystery - how to remove?

The spindle on my machine differs from the assembly shown in the manual. This includes the collet closer itself (hydraulic piston, drawbar, return spring etc) and the collet / spindle nose end. The machine came with a 3/4" 5C collet fitted, along with a spindle nose protector. 

Once removed, the protector reveals an "A2-5" spindle nose. That's got a tapered register for accurate radial alignment, combined with 12 bolt holes for fastening a chuck. I've ordered a backplate from a US supplier on ebay - the only vaguely affordable option I could find. I plan to try to create a D1-3 adaptor from it but first, I'm struggling to figure out how to remove the collet cartridge itself.

The spindle has a 2" bore, although the 5C collet cartridge itself has a max bore of 1.25". So I need to remove said cartridge otherwise I will be limiting myself on the size of stock I can hold.

Needs a bit of cleaning up but you can see the 12 bolt holes. One of them contains a drive pin and there's a 13th hole (at 12 o'clock here). That must be something to do with the collet cartridge surely?

It had a grub screw in it but that was simply plugging the hole (hence the visible thread here, which looks like 3/8" UNF). It seems to be angled down towards the centre, so perhaps some form of locking mechanism to hold the collet cartridge in place? It can't be for lubrication, as the grub screw isn't sealed (would spray oil every where) and the bearings are said to be greased and "sealed for life".

The cartridge has a stepped nose but on close inspection, it's clear that the cartridge is pushed into the spindle nose at the larger diameter.

Looking up its jaxi doesn't help much although you can see that the cartridge certainly creates a restriction. It needs to come out! The through bore is 53mm and the larger OD of the cartridge is 55.7mm or so, so clearly there is a step in the bore.

There's a secondary, smaller M6(!) thread at the bottom of the angled bore but there's nothing to pull out - and nothing like a push rod to be removed. It looks as if there may have been a push rod to lock the cartridge in place, held in with an M6 grub screw. It's always good to mix your threads - M6 and 3/8" UNF in the same bore! And no, it's not 1/4-28 UNF or 1/4-26 BSF.

The hole is at 30 degrees and is ~65mm long. Simple maths places the end of the probe at a diameter of ~40mm. Note that the bore of the collet cartridge is ~1.25" (31.8mm).

This is one of those puzzles where you have to figure stuff out without instructions. My next step will be to create some form of puller. Probably a plug that goes in from behind, combined with a large diameter tube at the front. Then a large washer and bolt to pull the cartridge out of the front. Surely there can't be any more to it than this?