Remove Old Grohe Showerhead Handle

[54fighting]

I believe this Grohe fixture was installed in 1996. The showerhead is leaking, and I need to replace the pressure balancing cartridge. For the life of me, I can't figure out how to remove the handle. There is a hole underneath further along the assembly (not under the handle), but it appears to have a nut in it rather that a screw or allen wrench hole. I thought that the top piece of the handle might pop off, but I'm not having any luck trying to pry it off and I don't wan't to mar the handle. I've attached pictures of the handle and the hole that I mentioned above. Any ideas?

 

[Terry]

I'm pretty sure it's a metric Allen. I've removed a few of those.

 

[Sponsor]

 

[54fighting]

I heard back from Grohe. The cap on the handle needs to be push up and off to reveal the restraining screw. I'm hoping I can do it without breaking/marring the cap; it's on very tight.