Verrrrrrrry nice!
I have your gun's first cousin, just completed putting this one together a couple months ago, closely duplicating mine as I carried in Vietnam.
1975 Colt SP1 lower and upper. I had to switch out/replace the buttstock, grip, sling, flash hider, and mag.
The 'waffle' mag is a period-correct non-functioning dummy, from nodak spud. All else is totally functional. Been to the range several times. Many favorable comments from fellow shooters.