1 1/2c Harding Imperf.pair - Legit?

Hi, is this legit? Note the 1984 postmark.....

Yes, a genuine rotary plate Scott 631 as evidenced by the wide gutter between the 4 panes of 100 stamps. (You have a "gutter pair")
As opposed to the flat plate Scott 576 which has guidelines between the 4 panes instead of the gutters.

Add: as for the 1984, they are still valid for postage today.

The Scott Specialized catalog calls this a 631 pair with horizontal gutter between, priced at $4.75 mint and $5.00 used. Note that it is valued more used than mint, however to get the premium price it needs to be a contemporaneous cancel, that is, from 1926 or thereabouts. Fun fact, that stamp's 100th birthday is coming soon, on August 27th.

High value Columbians and a bunch of other stamps are like that where the used price is higher than the mint (or no gum) price. You can't take a stamp with no gum and difficult resale value, apply a modern cancel, and get a "used" stamp that is worth more and has a bigger market than a no gum stamp, although people always try...