The symbols are individual inspector's marks and were applied as the gun went through the production process.
The stock is illegal because under BATFE rules only the proper stock may be attached to this Luger, i.e. an original or reproduction stock of the proper dimensions and configuration. Otherwise it is considered a "short barreled rifle" and subject to restrictions and licensing.
Free is good
. Take the iron off and store it away until you come up with the appropriate wood.