odd knob on AE type 81 handset

[Argee]

Just picked up a 183 space-maker on ebay.  I didn't see anything different about it till it arrived.  It's got an odd knob at the top of the handset.  it's numbered 0-9.  My initial thought was some kind of volume knob.  I've done some searching online but I haven't seen a reference to this knob.  The weird part is that it isn't actually connected to anything inside the phone, so it has no function.  Is it factory or a later addition?

[rdelius]

That was a amplified handset missing the guts