Statements of Cash Flows - USD ($)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
OPERATING ACTIVITIES
Net income	$ 14,507,936	$ 13,912,672
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	705,197	979,652
Stock-based compensation	93,360	40,920
Deferred income taxes	(31,269)	96,275
Changes in operating assets and liabilities:
Accounts receivable	(106,859)	548,023
Inventories	(614,437)	(292,141)
Prepaid expenses and other assets	(180,885)	(27,877)
Accounts payable and accrued expenses	(154,049)	37,137
Deferred revenue		(142,733)
Net cash provided by operating activities	14,218,994	15,151,928
INVESTING ACTIVITIES
Purchases of fixed assets	(68,265)	(604,800)
Purchases of marketable securities	(13,672,407)	(18,217,410)
Proceeds from maturities and sales of marketable securities	20,800,000	19,540,000
Net cash provided by investing activities	7,059,328	717,790
FINANCING ACTIVITIES
Proceeds from sale of common stock	217,940	51,040
Payment of dividends to shareholders	(19,374,040)	(19,365,040)
Net cash used in financing activities	(19,156,100)	(19,314,000)
Increase (decrease) in cash and cash equivalents	2,122,222	(3,444,282)
Cash and cash equivalents at beginning of year	4,755,082	8,199,364
Cash and cash equivalents at end of year	6,877,304	4,755,082
Supplemental disclosures of cash flow information:
Cash paid during the year for income taxes	$ 3,426,045	$ 5,966,425