That female threaded fitting is the hI PRESSURE fill nozzle. The Threads look new(clean) because this fitting usually has a yellow cap installed. You remove the cap to install the fill nozzle. Since NO yellow cap, then it has been lost. Obvious cause of no LP after fill is the filler did NOT fill the tank full. If it leaked out the usual 15 to 25 gallons of LP in a few days you would hear it and smell it. You need to get a cap to cover the fill nozzle. This link shows what it is. Doug
https://www.amazon.com/Marshall-Excelsior-ME109-Tank-Valve/dp/B007HRKQ2Q/ref=asc_df_B007HRKQ2Q/?tag=hyprod-20&linkCode=df0&hvadid=312191829731&hvpos=&hvnetw=g&hvrand=7850837316716751630&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9027230&hvtargid=pla-571603853261&th=1