There are two stages to making this mod; the first is that a "grub screw" needs to be fabricated to lock the anti-force dog permanently to the inner shaft;
V70_modified_anti_force_dog.jpg
I used an M5 x 0.8mm metric-course tap, in order to thread the existing sliding-pin hole. The hole itself is just the right size to tap from, although I did use a set of three taps to make this thread (first tap to begin the threading process, with a heavy taper, all the way through to a plug-tap). With care and a bit of cutting-oil, a final plug tap could be persuaded to go into the sliding-pin hole, I reckon.
The "grub screw" is a long M5 x 0.8mm screw, cleaned in solvents, and with its tip ground to a conical form. I ground a fillet of around 15degrees from the face of the end of the screw, just enough to allow the screw to wind deeply into the existing slot in the tumbler centre shaft - as the sliding pin would have achieved in normal use. The sliding pin itself is now no longer required: It is shown in the photo for comparison to the tapped hole only.
Before the modified anti-force dog can be fitted, the region of the side of the tumbler outer casing (as indicated in an earlier photo/post) needs to be cut-away with a Dremel, or similar, then dressed clean with a needle-file on the inside of the cut near where the "ramp" feature for the sliding-pin would have been - this is just to ensure nothing snags in service.
V70_modified_tumbler_barrel.jpg
Having dry-assembled and tried this modification, I re-cleaned the screw thread, applied some Loctite Studlock to the first 5 threads, then wound it in with the anti-force dog in-situ, ensuring that it was all locked wrist-tight before leaving everything to set for a few minutes.After this, the rest of the screw was cut-off with the Dremel cutting-disc, as close as possible to the dog face as I could get. Once the bulk of the screw was removed, I used the cutting disc to gently grind the remainder of the protruding screw flush to the dog-face (I know, I know, you should never use a cutting disc as a grinder, but meanwhile, in the real world..........). I refitted the circlip to the end of the tumbler inner shaft, just for completeness. It isn't 100% necessary, but just in-case the "grub screw" works slightly loose, it would be good to know that the dog-drive cannot slide upwards on the shaft.
A quick blow-down with the air line, followed by a trial run with the key in the tumbler-assembly, proved that it would all turn without snagging. At this point, I was able to wash the whole tumbler assembly in brake & clutch solvents again, blow the whole lot dry, then lubricate with a "dry PTFE" spray up through the key-slot.
At this stage, the spinning-key-syndrome should be fixed, but to ensure that the key barrel assembly never gives trouble again, I did some "belt & braces" modifications to the rest of the lock mechanism.