I have a bunch of rods, I bought a few sets of "240Z" rods on eBay and they all turned out to be 260Z or 280Z rods. The only true 240Z rods are the first set I bought. I needed more rods because one of the rods from the first set is flawed.
here's the markings on mine(X = any number) :
L24 rods : 51-0 (logo)JAPAN.NO142.X
L26 rods : XX-X (logo)JAPAN.A8742-X
L28 rods : 4-1(logo)A87-2
L28et rods : XX-X(logo)A87-2
Only L24 rods have "NO142", the others all have "A87" in the marks.
The other way to tell one from the other is from the form of oil squirt hole. L24 rods have "outtie", all others have "innie". The L24 "outtie" is a oil hole on top of a protruding lump of metal. Non-L24 rod's "innie" is a hole inside a egg shaped depression. Use this to identify the rods when you don't have any measureing tools with you.