ordered one from aim and not sure if I should add one for $5 in the option they give you.

If you don't have a micrometer sight, what is the other rear sight given?  a peep-hole or a u groove?

That is probably the milled Mk1 rear sight. Looks like the long range sights on Sharps rifles. It's adjustable from 100mtrs to 1200meters. Not adjustable for windage.

It could also be the stamped Mk3 sight. Same function as the Mk1 only stamped rather than milled.

The Mk2 sight was a dual aperature much like the M16a1 sight. Same sized hole but one for short range and one for long range.

For $5.00 more I'd go for it. I would also expect the stamped sight. The milled usually sells for a bit more nowadays. But you might get a milled sight.