Inventories - Summary of inventories (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Inventories
Raw material and common parts	$ 16,677,000	$ 16,989,000
Work-in-process	3,587,000	4,273,000
Finished goods	3,770,000	3,864,000
Inventory, Gross	24,034,000	25,126,000
Less inventory reserve	(1,777,000)	(1,720,000)	$ (1,720,000)
Total inventories	$ 22,257,000	$ 23,406,000

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Amount before valuation and LIFO reserves of completed merchandise or goods expected to be sold within one year or operating cycle, if longer.

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Gross amount, as of the balance sheet date, of merchandise, goods, commodities, or supplies held for future sale or to be used in manufacturing, servicing or production process.

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Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

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Gross amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed.

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Amount of valuation reserve for inventory.

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Amount before valuation and LIFO reserves of merchandise or goods in the production process expected to be completed within one year or operating cycle, if longer.

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