Congrat's with your new JD x300 addition!
I ordered the 42"M dedicated mulch deck with my x300. (no discharge opening)
Walked from the sales area into the parts dept. to buy mower blades. Counter parts guy informs me the 42M deck blade set has a left blade and right blade. The blades are stamped with the word left or right on the bottom side of the blade. I was thinking, just one more thing I can goof-up by installing the mower blades reversed. LOL!
FYI: I modified my X300 to mow in reverse without lifting and holding the RIO lever. requires only 2.5" insulated wire and 1 wire tie. The modification is easy to remove if I were to need JD warranty or service work performed. The modification is added at the electrical wiring plug for the RIO switch.
MY wife hates the RIO switch pull and hold,to cut in reverse, feature! She would backup and forget to pull the RIO to mow in reverse. The cell phone rang, the wife said. "Fix this (@#%&) mower." The old JD STX38 didn't have the RIO reverse mowing feature. Anyway, the wife's now happy with "her new mower". Life is good!