Just a guess on my part...but maybe it is used to distinguish a model with a Slide Out (SO) versus one without...or versus one with dual slides (DS)?
Oh...and maybe 3512 is 35' long & 12' wide (with slide out extended).
In case you can't tell...I'm bored tonight!
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2014 Wildcat 312QBX
2014 Ford F350 SD 4X4 CC LB 6.7L PSD
Bob & Debbie, USAF Retired