almost 17 year old 20055 PP and on it that rubber has only been replaced once. Your mower has six different variants based on serial number. Without that no one can look up the part. Mine originally came with one with a slight curve at the end but the replacement #120-7011, which replaced #107-7496, does not. The curve is what damaged the original many years ago. Yours might be #139-6597. Mu mower is a 21 inch, yours may be a 22 inch. I don't go backwards a lot and try to mow in a forward direction. Also we mow on "C". If the mower was set lower like "A" or "B", I could see that being harder on the trailing shield than a higher setting. I have tall fescue and bluegrass that is kept at 3" in summer. You say you have St Augustine so you may set it lower to keep the St Augustine from becoming a mat. My late father had a twin of my mower and he used it on St Augustine. Never had a problem with it. I did look up the 139 part I found listed on one of those six variants and it looks just like mine but probably a little wider, e.g., 22 inch mower vs 21 inch mower. Maybe set the mower a setting higher and try to go backwards less?