No mystery.....that was how grease nipples used to be. The gun just has a cone to match the nipple.
Back in the day, before motor vehicles had maintenance free suspension joints, it was a regular maintenance task to grease these suspension joints frequently. The joints were generally unsealed, no rubber boots and the grease attracted road grit, and along with water, salt spray etc they wore, so only low pressure was required to grease the joints as the internal clearances opened up. The cone connection was adequate for these simple type of joint.
Then came progress, non return valve in the nipple, and sealed ball joints, all requiring higher pump pressure. The push on cone was barely adequate, useless in most cases and the clamp on design became the new standard, so all you need is either an early cone type gun to match the nipple or a modern nipple to fit your gun.
Regarding the problem with the gearbox cover, thin wall bushes can be removed by carefully splitting length wise with a small hacksaw, just cutting enough to weaken them to allow them to be collapsed inwards. I would use heat on an alloy casting to aid insertion of any bush. Big pressure and old castings always carries the risk of cracking. Chance the casting was cracked when the bush was inserted, and being non standard, the interference fit would have been more likely by educated guess rather than by calculation.
Swarfy.
