I need to correct my evaluation of the Davis Mk3 to mention that there are no pinch tabs to lock and unlock the index arm. I rely on that to hold my reading while I look at the time. The more delicate and bulkier Mk 15 might be more accurate because it does have pinch lock tabs. I have one so I actually know.
I also noticed that there is not a complete set of filters for the horizon sight on the Mk3. This is fine for sighting with a sea horizon, which is never blinding.
But if you are on land using an artificial horizon reflection you need full protection from the sun's reflection as well, so it would not be good for land based practice unless you can rig up some more filters for the horizon mirror. It's nice if you can balance the brightness of the reflection and the sun. The Mk 15 has both sets of filters.
Knowing all this, I just purchased a Davis Mk3 anyway, hoping I'll find that it is either tight, or I can make a way to hold the arm fairly tight. I feel confident I can add more filtering on the horizon mirror.
By the way, for an artificial horizon I use a cheap aluminum (foil) cookie sheet, got two for $1.69. I pour in some used motor oil and it doesn't freeze at zero degrees. I've found that when sighting a low body a longish pool aimed in the general direction is best. I set the pan on a very cheap plastic tray type folding table. I use a cover from a plastic tub to set over it and keep out dirt and snow, couple small split firewood pieces holds the cover down. My "observatory"
If I find I really like the Davis Mk3 I may put my Mk15 on eBay. If I do I'll try to remember to post a link here so anyone interested gets a heads up. Either model would be good to learn with. In fact, I think I could get rough data with a plumb bob and protractor, if I had a helper to read it or held the string with my left hand.
In the northern hemisphere Polaris alone gives pretty good latitude. Think of the Vikings and others in the old days, there is lots to this stuff and it's fun to learn. Without chronometers they used to find their latitude and cruise down it till they reached their landfall. Dangerous at night though.