Sorry your Dad got "taken", in spite of other opinions, a .30 cal luger with the stock lug ground is worth at best $600, IMO.
Nice 1920 commercials can be bought for $1000 - 1200 and sometimes a little less.
As far as info goes, you are on the correct site. Just do a site search for 1920 commercial and you will get dozens of threads to read.
You ask "simple" questions, that it takes hours and maybe years to learn! Some clues to the id of your pistol:
the crown/N is a commercial nitro proof, commercial pistols are not dated, the serial number suffix helps to date the pistol,
military pistols(most, but not all) are dated on the top of the chamber and are 9mm caliber.
Removal of the stock lug takes the pistol out of "collectable" status and into shooter status, but .30 luger is expensive as was pointed out and not really much in favor for shooting; but if you can find the ammo it is a pleasure to shoot.
Go to an online book seller and search on "luger" you will find offerings of many books. The cheaper ones are not really good for collectors, but may be of interest to you for general knowledge.
I'll echo the comments of others, if you know the physical location of the seller, and he mis-represented the luger when he sold it as it appears, take it back. Your Dad's background should be sufficient leverage to achieve a satisfactory outcome.
In this case the "ignorance of the seller" is not a valid defense, IMO.
Hope this helps, and welcome to the luger world- of "research first, buy second".
03man - Don Voigt
Luger student and collector
Looking for DWM mil.side plates- 14,17,45
Art. luger rear sight slide # 03