Are these stock wheels on 1973 Mach 1

those are Kelsey Hayes Magstar wheels that were standard equipment on the 69 Mercury Maruader X-100. these wheels were also used on the 67 Shelby Mustangs except with a chrome outer rim instead of the trim ring. i do believe they were available as dealer installed add on for pretty much any Ford product back in the day and the early 71 Boss 351 promo pics showed a yellow Boss 351 with these wheels but again with the chrome rim instead of the trim ring.

they are a good looking wheel and lot's of folks like 'em, including me. they are fairly popular with some of the early Cougar guys as well since they were optional on the Group 2 Cougar package that Carroll Shelby sold back in the day, which was also available from your local Ford dealer. if your brother gets the car and wants to sell those wheels i'd be interested, provided i have my mustang sold by then so i can actually afford them

BTW, if he's interested i have a 69 GT coupe, mild restomod, for sale right now. i'm asking $28,000 for it right now but that is negotiable. i have an ad in the classic classified section.

They look like Magnum 500's but not quite...... Magstars, maybe? There were wheels like those put out by both Ford and GM... The main difference that I can see between the Ford Magnum 500, the Oldsmobile SS1 and the Chebbie SS396 is the bolt-circle diameter.
Magstars -and the rims on that Mach- have longer (side-to-side) slots between the spokes than a Magnum 500.

But, I think, Ford sold Magstars at one time.........