Installing propane tank to gas range with strange copper pipe
I am going to "go out on a limb" as we used to say and tell you that the shiny flex gas line in the bottom picture is used for natural gas only and the copper line is the one for the propane connection. Any time I hooked up a bottled gas device to a bottled gas source the tubing to the appliance was hard piped or done in flexible copper with flare fittings at the connection point. Compression fittings were never allowed. You will probably have to remove that shiny flex piece up to a threaded fitting, and purchase or add the required fittings to adapt the copper tubing to the device (stove). The copper line looks like 3/8" OD or 1/4" ID (they are the same thing). To complete the installation, you may need someone that can "flare" the copper tubing (that would be a propane dealer or an HVAC company). Stoves in my day, could utilize either propane or natural gas with a minimum of adjustments. Yours is probably set up for natural gas.  If that is the case, nake sure it is changed or adjusted to the fuel you are using. I hope this helps.