Hello Craig,
In V. Bobba's book on Swiss lugers on page 126, he mentions that at or around pistol # 26205, the rust bluing technique with frame interiors "in the whilte" gave way to immersion-blued pistols with their interiors blued.
Regards,
Pete... <img border="0" alt="[typing]" title="" src="graemlins/yltype.gif" />