Thanks Egronvold.
Time to step away from the ZX and tackle the barrier lift mechanism. The full scale looks to operate with a bi-stable latch which allows the jaws to engage every second plunge. Well suited to RC play with no electronics needed. Completed the CAD work to carve a path inside the housing. A 1mm spring pin is what follows the groove and keeps the plunger from falling out.
Parts look great and assembled well, but the latch only advances to the next position on rare occasions. Everything is internal, so there is no real troubleshooting i can do. Full redesign is in order