Yes, that looks like a Nivomat unit. They’re not just a damper, there’s also a spring inside it. It you use a standard damper with the same spring you used with the Nivomat, the rear will definitely sag.
Your choices are as follows
Find a pair of good Nivomats (not that easy to find good secondhand units, and likely to be expensive) or
Fit a new pair of regular springs. (Standard, or heavy duty)
A quick search shows these
https://www.rendcarparts.com/parts-f...927/RD9131927S