Most of the higher end fly lines have the line codes stamped onto the fly line. No all of us buy high end fly line. Here is what I do….
Take the spool off the fly reel, and with a metallic pen, write the fly code inside. Nobody can see the mark. It isn’t affected by weather and there is no wear in that area. If you decide to remove it, or change codes later, the metallic will come off with the correct solvent. I blocked out the model and brand on purpose.
What if you use more than one spool on the reel? Looks like you are writing on the reel part.
If you have more than one spool for a reel, you could try writing on the spool instead. It would be a moving part, but I don’t see it being a problem with wear by friction.
One trick I learned..use a long mark from a sharpie to equal a 5 and short one to equal a 1. For an 8wt line, one long mark and 3 short ones.
I’ve done this for years as soon as I put the line on the reel for the first time. Put the marks near the front loop. Refresh the marks occasionally with black Sharpie as needed due to fading.
Sometimes I mark the line itself with a sharpie. A long mark stands for 5 line weights and short marks stand for 1’s. So a 7 weight line would have one long mark and 2 short ones.
Those line codes can be a little hard to read!
Where on the line would I find the line code?
Let me send you an email with a picture, as I don’t want to identify brands here.