I have an LA135 and had read where others have had this issue as well...where the belt rubs a section out of the belt guard. Apparently it is a design flaw from what I understand.
I've run my 135 for about 7 hours, but at about 4 or so, I decided to give the deck a good once over and found that after only 4 hours, my belt guards did indeed have these areas rubbed away by the belt.
I figure that the damage is already done, and now that the grooves are worn, there's probably not much to worry about.
There had been a thread, either here, or TBN, or Gardenweb about this subject. I really don't remember which forum it was. You might do a search on each for LA135. If I remember, they didn't include the SE part in their title.
You should likely let your dealer know about the problem, and have it documented just in case of further issues.
Yes, there are two known solutions provided by Deere to help resolve the problem.
One involves placing a standard metal belt guide (L-shaped with dimple) about three inches in front of the plastic shield (in line with belt),and the other solution involves removing the plastic shield altogether and substituting it with three of the L-guides.
Both procedures require drilling obviously.
Contact your local Deere dealership and inquire about the installation refering to DTAC number 79313 if the conversation with them doesn't jog a memory.
I know this thread is old but my 135's belt has also worn away the plastic guards. I sent a email to Deere and asked if they would replace the belt under warranty if it fails prematurely. I will update the response they send.