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In days gone by, the verb "to show" was expressed in third person as "he has shown."
A more recent update of the same work has the verb construction "he has showed."
Is this an error in transposition, or was there a change in English grammar at some point?
I think this was an error. The following are correct:
Present PerfectPast Perfect I have shownI had shown you have shownyou had shown he has shownhe had shown she has shownshe had shown it has shownit had shown we have shownwe had shown they have shownthey had shown
Question re Present Perfect of verb "to show" 
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