The oxygen turbopump in the RL10 upper stage engine has been openly discussed by NASA for some decades now, in publications like SP-8107. On page 8 of this document is a drawing of this turbopump. The lower end of the oxygen pump shaft seems to be splined.
In the back of my head, I seem to remember reading somewhere that it has something to do with a possible usage of the engine in an ICBM, but that doesn't make much sense, considering it uses liquid hydrogen as fuel.
Can anyone tell me if it is indeed splined, and what is supposed to be connected there, if anything?