When I looked up the part number for  the piston it shows
a 251 prefix. A lot of these pistons on the auction site show
to fit the AT1 and the CT1. This can't be correct can it? Looking 
to buy a first over and rings. There is also a reference to a 248
prefix.
 

248 prefix is for the AT1 125 pistons  and 251 prefix is for CT1 175's. Many e-bay sellers don't know anything about the parts they list or what they fit and just list the bikes they see in the related fitment list. Yamaha includes both bike part numbers in the same parts diagram some times. Same on some DT250 and RT360 listings. Don't ever rely on a seller to list a part fitment correctly. This goes for buying off partzilla or other parts supplier too. Best to know your part prefix for the bike you are needing parts for before ordering parts.   

Thanks, I'm more dialed in on the 250-360 numbers.

Indeed. Once came across a nos piston advertised for a DT175MX. Part number on box showed it was for a 1972 CT2/3

Number prefix 314.

Sent a message to put the seller right, but over a month later it has still been relisted as a DT175MX piston.

Same piston kit might do both as both 66mm 175 reed inlet etc. Here's an ebay link to a wiseco kit recommended for all those models.

eBay item number:285059459749
www.ebay.com/sch/ebayadvsearch?_sofindtype=4