MG T-ABC

As it’s time for routine maintenance on my TC & having squeezed more grease into all the grease points, I thought I’d try a previously unused push type oil gun to lubricate the tacho drive on the back of the dynamo. However, when I took it out of its box it had come apart & I can’t work out how to reassemble or if perhaps I’ve got a missing part.

Pic attached - any advice would be much appreciated. BTW the spring is loose - I’ve just assumed it fits in the U shaped tube as pic