The P88 being a "failed" German service pistol due to price only comes in 9mm... however there was a 9x21mm barrel available.... I think Walther sold the 9x21 only as a "extra barrel" and not a complete pistol
The P88c comes in 9mm, (4inch and 6 inch barrel available) the above mentioned 9x21 and also in .22lr
some of these came with adjustable rear target sights as was there P88c's with matching serial#'ed conversions as a set