Buck tags are behind the driver-side inner marker lamp and headlamp. The '82-'86 tags are up higher and easier to spot than the '87-'93 tags.
You VIN is in a group of 86 consecutive VINs and your DSO is 83-0114.
83 = US Government (General Service Administration, or GSA)
114 = domestic special order #114
There have been two other DSO 83 SSPs found in the same group of 84 VINs, one before yours (83-0113) and one after yours (83-0115). The one before yours is unknown which US Gov't office used it because we have no paperwork. So I'm afraid unless you have some paperwork to go with your car, from when it was originally ordered or when it went to auction, that states which US Gov't dept it was ordered for which dept used it, US Government use may be as close as you're going to get.
We know the one after yours was used by the FBI because auction paperwork indicates such.
http://www.specialservicemustang.net...info191668.htm