It is not unusual although rarely used these days unlike, as I recall, in the 50's and 60's. It was used as a formal address for Untitled Gentlemen. Letter would be address John Smith Esq. and letter commence Dear Sir. Sir used, as it still is, even though the person is not Knighted as in Sir John Smith. Very commonly used years ago even on a letter to a young boy. Just polite form. Maybe the practice should be revived.
A copy of Debrett's Correct Form from Amazon or wherever will explain all. You'll be pleased with the purchase next time you take tea with the Bishop or attend a Royal Garden Party. :-)
https://www.ebay.co.uk/itm/37459711...msclkid=e076cf524a5111917964abc3e4233b0f
