I checked after Robert's post.
The verso page states First published 1986
Reprinted in 1989
That would suggest the dust cover remains similar in the two printings.
It does not suggest being updated.
Query: if a book is updated, would the ISBN number change
Since my edition is the 1989 production, and the dust cover inscription is "1st Edition" then I guess those two printings are identical. (1986 and 1989)
Was not aware of the 1991 issue.