Proxar lenses were also used on plate/large format cameras to shorten the focal length as a cheap wide angle accessory. Zeiss also made Distar lenses with negative power, that made the focal length longer.
I find that it obvious in the finder that the Xenar on my 'cord gets wider when I use a Rolleinar.
Using a +1, the focal length changes from about 75mm to about 70 mm and about 65 mm for the +2. This is not dramatic, but noticeable.