Case 27: Přerov district?

This is Přerov district (okres), 1960–2001 system. PR = Přerov (North Moravia). White background with black text = passenger car. The "E" is likely an additional series letter — as the original 4-digit number space (PR 00-01 through PR 99-99) filled up, districts were allocated extended series. So the full reading would be PR-E 90-25, meaning Přerov, series E. That said, if this is from a degraded negative and the "E" is uncertain, it's worth noting that PR as a two-letter code points unambiguously to Přerov regardless — no other district shared that prefix in the 1960–2001 system.