The previous owner most likely wanted to get rid of the lambda (O2) sensor moving back to the old version (I.e. Pre-2007 model). If it works I would suggest you leave it like that. You can better manage the fueling and eventually avoid the famous surging. By the way what is the last digit of the ECU number?

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that is how i feel about my XTR
Mods so far:
Hyperpro front springs; Hyperpro rear shock spring; modified air box; LED tail light; LED indicators + new relay; YZ pegs; Lucas aluminum bar; brisk spark plug; snorkel removed; DNA stage 1; second hand DynoJet O2 mod; crash bars; front and rear steel braided brake lines; soft EBC brake pads; mitas 07 tyres, 2in1 Metal Mule exhaust, Blocked AIS, tall OEM screen, OTR bash plate