It is a lens that screws into the front of a lens and not a rear teleconverter.
from Sigma's website (
https://www.sigma-photo.co.jp/english/new/new_topic.php?id=496):
Conversion Lens FT-1201 for SIGMA dp3 Quattro
We are pleased to announce the development of Conversion Lens FT-1201 that is exclusively designed for the SIGMA dp3 Quattro.
When it is attached to the front of the lens, it magnifies the focal length of the lens by 1.2 times (60mm / equivalent to 90mm on a 35mm format), and does not change the f/2.8 aperture value. It consists of 4 ultra-high quality lens elements in 3 groups, and maintains excellent rendering performance while offering a different focal length and therefore more shooting possibilities.