disc brake mounting problem

[bitt3n]

I bought a used set of hydraulic calipers, the rear being a BR-RS785.

http://i.imgur.com/XTHvvkA.jpg

However, this appears to mount to frames with the screws going down into the chainstay, whereas my frame appears to a require a caliper that mounts horizontally (https://www.sevencycles.com/options/low-mount-disc-tab.php).

http://i.imgur.com/AMXphzO.jpg?1

Is there some way to mount the BR-RS785 caliper onto this mount, or was I supposed to have purchased a different caliper? (Obviously I should have checked the mounting bracket.. I didn't realize there were multiple standards.)

[sales guy]

The brake you bought is a post mount. The style of mount on the frame is an ISO.

You need an ISO to Post Mount adaptor. They are like 10 to 15 bucks.

[bitt3n]

so is this (https://www.amazon.com/RavX-Disc-Brake-Adaptor-Post/dp/B0060ZB4TA/ref=sr_1_fkmr0_1?ie=UTF8&qid=1496159311&sr=8-1-fkmr0&keywords=iso+to+post+140mm) the correct part? The rotor is 140mm.

[ColonelJLloyd]

Yes.

[sales guy]

so is this (https://www.amazon.com/RavX-Disc-Brake-Adaptor-Post/dp/B0060ZB4TA/ref=sr_1_fkmr0_1?ie=UTF8&qid=1496159311&sr=8-1-fkmr0&keywords=iso+to+post+140mm) the correct part? The rotor is 140mm.

Yep. That will work. There are a ton of companies offering adaptors. Shimano and SRAM both do as well.

If the rotor is 140, make sure to get the right mount. most mounts are 160mm as that is the most common rotor size.