I rescued a Colt Python 4" blue with serial # 548xx that makes it a 1966 model from a local pawnshop. It's about 75- 80% shooter grade due to a lot of holster wear on the cylinder,front of the barrel, and trigger guard area. No pitting, just the bluing worn down to bare metal in certain places. It came with rubber grips, and it locks up tight. All in all I'm very pleased with it as it'll made a solid shooter. In addition, it has a hollow lug barrel and Ellison rear sights. while doing my bring home clean-up inspection on it, I found that the left side of grip at the bottom had INS factory stamped on it. Does this stamp indicate that this is an old Border Patrol revolver? I pulled the grips on my 1981 year model Pyton and it did not have this stamp. Does anyone know if Colt Pythons were issued to the Border Patrol during the mid 60's? Thanks in advance for any info.