According to the Vademecum, "Lenses also marked ROJA or Rath Opt Ind Anst. for Rathenow Optical Industry Anstalt with a I like a "J" in old script. This trade name began in about 12/8/1898 (Amateur Photo 12/08/1898, p644) but was found not to be a success and later the firm reverted to Busch, but the date is not available here. It seems to be Emil Busch in 1901 adverts. Accordingly, I suspect this one dates from about 1899 having both. It looks like the Aplanat shown in a clipped advertisement which also shows both diaphram and waterhouse stop versions being avaiable. This is the Waterhouse stop version, but you will need to make your own (which isnt too hard). it measures about 44mm across the mounting threads - no flange.

Brass looks great - likely cleaned at some stage, you can see some separation on the very edge of the front element . It looks like it will take great photos

4.5cm across front, 6cm long.  44.7mm across outside of mounting thread.