There are two versions of locking lever that I have found on Rexall Monogram pens. I found three Lotz patents. Two seem to be the two piece locking lever on the black pen shown below, and one appears to be the one piece spring action locking lever on the MHR pen. For filling, it is a standard lever. However, the lever locks down securely to the barrel. The mottled pen has a one piece lever in which the end compresses on a spring. You push in slightly on the end of the lever to release it, then lift normally.
The two piece mechanism seen on the black pen can be more confusing. Do not try to lift it! You have to slide the far end piece (which is the locking mechanism) just a mm or two, and then the actual lever is released. There is a small slot in the locking piece so you can use your fingernail to slide it.