Hello TEA,
I hope, you can help me with a thing that confuses me.
A few months ago I have bought a nice HP 3456A.
Here are some pictures of it:
Top view of the digital part:

Top view analogue part:

Mains input, no RIFA surprises:

The power supply unit, top view:

El Capo:

The input amplifier:

Some input protection:

A fancy standoff party:

A funny thingie, no idea what this is, looks like a wirewound resistor:

Another HP special item:

And this is the master of all this desaster, the reference:

But now to my question, hopefully some 3456A experts are around.
The schematic for the oscillator crystal states the following thing:
Mains input 60Hz - 5.85MHz
Mains input 50Hz - 4.875MHz

But when I'm looking at the crystal in my 3456A I found a 6MHz crystal:

It looks geniune and untouched.
Can somebody explain this to me? I'm a bit confused.

I have ordered some 4.875MHz crystals from this .cz company
http://www.krystaly.cz/en/Can I replace the existing crystal with one of the 4.875MHz?
Thanks for your answers.
Mounty