Well isn’t this kind of curious? The newer Gigaset S79H handset is being offered on Amazon for $69.95. In contrast the older S67H handset is being offered for $63.08. Both are expansion handsets intended to be an addition to an existing Gigaset systems. They’ll work just fine with either the A580 or S675 base.
The difference in price is very slight given that the S79H actually has some of the features of the SL78H handset, which is offered for a whopping $149.95. The S79H has a 2.5mm standard wired headset connector like the S67H, but also a mini-usb connector for use with the Gigaset QuickSync software. That means that you can conveniently sync up to 500 contacts from Outlook. It also has a handy alarm function and rational volume buttons on the side of the handset.