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how exactly does 'identify' work?

One additional point on your original post.  You added row names to the test data frame, but did not specify the name of the data frame when you did the regression, rather you attached the data frame.  When you did this lm found x and y, but did not find the rownames, so the diagnostic plot just used numbers to label the extreme points.

This is just one of the many pitfalls with using attach rather than the more direct methods, try your example again but instead of attaching the data frame use it in the data argument to lm:
Then when you do plot(test.lm, 2) the most extreme points (3 if you don't change the id.n value) will be labeled using the rownames.