RD In view of your findings, I would guess it is for changing valve stem oilseals.
Valve held shut, by rope down the plug hole trick or compressed air fed with plug hole adaptor. Compress the spring slightly, tap the valve cap to release the collets, move the spring down to clear the collets, and if all goes to plan, collets out, spring can then be removed to reveal the seal.
As long as the collets can be removed through the window, it will work. Presume small tool to get into a confined space.
Really a two handed job for the sake of ease of operation, compressing, tapping, picking, holding....searching.
That's my guess, any other ideas?
Swarfy.