Note 7 - Income Taxes - Reconciliation of Income Tax Provisions (Details) - USD ($)	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Tax expense at U.S. statutory rate	$ 6,282,771	$ 6,645,733	$ 6,272,341
State income taxes, net of Federal benefit	103,240	83,116	118,057
Domestic manufacturing deduction	(495,361)	(536,063)	(476,932)
Municipal interest	(1,378)	(4,207)	(4,171)
Tax Reform Act effect on deferred tax assets	206,693
Other	187	(41,437)	(24,513)
Income tax provision	$ 6,096,152	$ 6,147,142	$ 5,884,782

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