This is the 8.3˘ Ambulance stamp from the Transportation series. These two issues are more difficult to tell apart.
Scott #2128 was issued in 1986. The “Ambulance 1860s” inscription is 18 mm. long. This issue comes in two varieties. There is a standard, unprecancelled version. Scott #2128a is precancelled by the Bureau of Engraving and Printing. “Blt. Rt. CAR-RT SORT” is in black ink.
Scott #2231 has an inscription that is 18.5 mm long. The .5 mm difference is so tiny, you need a millimeter scale to tell for sure. To me, I can’t tell the difference just eyeing them up. Scott #2231 only comes in the precancelled version.
If the stamp you have is not precancelled, it must be #2128. If the stamp is precancelled, it could be either one. Only measuring with a millimeter ruler will tell for sure.