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Costs and Estimated Earnings in Excess of Billings (Details) - Contracts Accounted for under Percentage of Completion [Member] - USD ($)
12 Months Ended
Apr. 30, 2020
Apr. 30, 2019
Costs and Estimated Earnings in Excess of Billings (Details) [Line Items]    
Revenues $ 38,000,000 $ 45,300,000
Anticipated Loss on Contracts $ 2,400,000 $ 136,000