"ctg" just stands for cartridge, as in 38 s&w cartridge. it has nothing to do with the model of the gun.
the old top break modelsof these are very common. if thats what it is, just buying one spring for it would likely cost more than the gun is worth. i hope it has sentimental value, and hes not paying you for labor, and he dosent intend on ever trying to actually shoot it