Marantz 2240 B Maintainance

I have a Marantz 2240 B receiver that seems to be in very good working condition, which I've started cleaning and have 2 questions about. First, I have removed the front panel screws in the 4 corners. What else is holding the front panel on? There seems to be something anchoring it right in the middle, around the "mids" tone control. Secondly, how do I clean the balance slider? Is it OK to just spray a cleaning solution into it? Thanks for your help!

Post removed 
The knobs must be removed and I believe you will find a nut holding the face plate on that's on one of the control shafts. Be very careful. If the slider isn't noisy, do not clean it. You can spray the slider dampening lube into the control itself and then it will get noisy. Also festoon bulb replacement (dial lamps) is a royal pain in the butt.
Thank you! You've confirmed the brass nut in the middle that several people told me about. I'm fairly certain all the bulbs are functioning, and I'll have to double check how the slider sounds. Thanks again!