If it wuz me, I'd take a piece of that pipe anywhere they'd have a caliper and measure the inside diameter of the pipe.
Type K has an ID of 0.527 inches, a wall thickness of 0.049 inches and an OD of 0.625 inches.
Type L has an ID of 0.545 inches, a wall thickness of 0.040 inches and an OD of 0.625 inches.
Type M has an ID of 0.569 inches, a wall thickness of 0.028 inches and an OD of 0.625 inches.
Or, contact the company that made the stuff and ask them:
Finally, the ASTM Specification that established the Type K, L and M tubing sizes was ASTM B 88. There is also ASTM B 698 that lists other standard tubing sizes, but ASTM wants to charge $38 to tell us what's in that standard. If you pop down to your local engineering library, they should have a copy of the ASTM Standards, and you can photocopy ASTM B 698.