Hello,
I bought a new shower head for my newly purchased home. The old head is the standard aussie shower head, which connects to the adjustable shower arm via a piece of hardware and a horizontal wing nut and bolt. See pic below.

The new shower head seems to be made to connect vertically to a fixed shower arm, and seems to have a different type of connection to the piece of hardware that goes between the head and the arm (no tiny flat head screw/pin and looks bigger). See pic below for side view of the hardware (attached to old head) and both shower heads)

Is there any way to attach it to the existing shower arm?
I was assured by the in-store team member when purchasing these new heads that it would be as simple as screw off the old head, screw on the new. Unfortunately that doesn't seem to be the case.
The only potential fix I've thought of is to buy a new piece of hardware to join them, but so far I have not been able to find this piece (circled in red)

Appreciate any help, thank you.