There's another system update, just available today or yesterday -- a "supplement" to 10.13.2. After you install it, if you click on the apple and check system report/system, the system version shown should be
macOS 10.13.2 (17C205)
At least, that's what mine shows and I am running on a late 2014 27 inch Retina 5K iMac.
On a different subject, one can use an iMac as a monitor by putting it into "target display" mode. Unfortunately, you cannot do this with a 27 inch Retina imac. I have no real reason to buy the new iMac pro but I would be tempted if I could use my current iMac as a second monitor, but you can't. There no real reason for this so far as I can tell as the iMac pro can support TWO 5k external monitors, and I see a fair amount of whining about it.