There are two things on the front.

An hole with a metal rod in it [perhaps a connection for something]

A red piece of glass with something in it. [stupid guess says it's an charging indicator.]

What are these things?


When looking for information about something, it always is a good idea to do a quick research in the manual. The 600EX-RT's manual shows on p. 6:

Slightly edited and cut version from manual, p.6

So my guess is that you mean the external metering sensor and the optical transmission wireless sensor, which both sit just above the AF-assist beam emitter (that would be my third guess).

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