If you're looking to bring a classic filter to life in your modern eurorack setup at minimal price, Behringer provides the way. Arguably as high quality or better than any offerings from Roland themselves, who have frankly ignored the Eurorack market at their own loss. Behringer is committed to reviving the classic Roland and Moog modular synthesizers of the 70s and making high quality, modern, careful recreations of all of their models and more.
Getting your hands on a real 121 Roland module in working condition from the 70s, provided you even have the right place to rack it safely, will cost you upwards of $1000 and not be different in any noticeable way except perhaps that single unit's own wear from 50 years of being on this earth.
Roland's own revival would be their, confusingly renamed "SYS-521" Eurorack module and frankly, the look and feel is less like the original, and it costs 5x as much as this Behringer unit. Save yourself the money - if this is the filter you want, go Behringer..