Hello Oakie,
Dating is problematic on 45s, because no factory records survive that
might have assisted. Unlike Briggs, the Kirbys have no known dating
system (based on model and serial).
However, we can get close. You have a 17".
1968 - 1972 (approx) Kirby with single rail
1972- 1976 (approx) Kirby with twin rail
After this, SB went to an all Briggs range, perhaps because Kirby
had ceased production of these engines.
Hope this helps.
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Jack