Hello,
Your gun has the following information posted in Gordon Herigstad' s, Thompson Submachine Gun Book. These two volumes have the most thorough information on all 15,000 Colt Thompson Guns. Your gun has a smaller, First Type Compensator, which is correct for the sale date of the gun. All Colt Thompsons were made without compensators and had them added.
You are correct the rear grip has been replaced with an M1/M1A1 series grip. These grips are configured differently than the 1921/1928 style grips. It also appears that you front grip has also been replaced, the color of the grip and configuration are not correct. The butt stock color appears okay and it should have a small "Anchor" cartouche on the front of the buttstock just below the metal piece of the locking mechanism. If it is not a Colt butt stock, but WWII made, this also affects the value.
It also appears that you gun has been refinished at some point, it appears to be Parkerized. The original finish of the guns was blue. The internal parts look all correct. It is a nice early gun with the earlier Automatic/Semi-Automatic markings.
Your gun, due to the refinish, is a shooter grade Colt Thompson. The value is somewhere around $25,000,00 to $30,000.00. This is due to the missing/incorrect parts and incorrect finish.