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Possibly a Level 4 city. When sacked, it gave 40,000 shmuckers, whereas when Kona was sacked, it only gave 20,000. Since it is unlikely that Progrock was a Level 5, this would indicate that it was either a Level 4 or there are modifiers to sacking which we are unaware of.
Real World References
The name is a shortened version of "progressive rock".