I purchased a used S&W M&P .45, black stainless with the numbers 13 /44 in gold lettering, and there is a lightening bolt between the numbers. I have the original case, manual and paperwork, but nothing describes what the 13/44 represents. The mags are 10 rounds and the barrel is 4.5 so that would not be an indicator. I couldn't find anything on S&W's website so I thought I my fellow InGun owners. Your thoughts please?