My understanding is that Turbo's were added to some D6's for altitude adjustment. I believe they were usually AG tractors. Seen as this tractor is located in Oregon, Which I am fairly certain has some altitude. At the National show this year was a fellow that had a D4 with a Turbo pulling a pretty large(IMO) Scraper. They do exist.
According to my rack setting manual only the D315 Industrial Engine 52B1-up had factory turbo charger. Either standard or "normalized" for higher elevations. Got up to 2000 rpm intermittent with 100 hp. Only built 372 of them.
When I was a kid the neighbor had a D4, no idea what series, it may have been an RD4, it was old, had the old overhead cable setup. This guy & his son were kind of the Red Green-type "innovators"...They were at the junkyard one day & there was an old turbo laying on the ground...brought it home, cleaned it up & scabbed it onto the D4, above the hood even...I guess it kinda worked, I don't think they changed the fuel setting, the ranch was about 7000 ft., so it just cleaned up the smoke...I remember it sounded pretty cool, first turbo whistle I ever heard, I suppose.
It says it's in Oregon in the lower ad, but under shipping it says it's located in Sanford Florida.
I also noticed the seller has zero feedback. It may be fine but I went through some nightmares with a zero feedback seller one time.