$ puttygen --version
puttygen: Release 0.70
Build platform: 64-bit Unix
Compiler: gcc 7.3.0
Source commit: 3cd10509a51edf5a21cdc80aabf7e6a934522d47

$ ~/Downloads/putty_src/putty-0.70-2019-02-18.5dadbdf/puttygen test_rsa -o keyname.ppk
Enter passphrase to load key:

$ ls
authorized_keys  id_rsa.pub   known_hosts      config
id_rsa           keyname.ppk  known_hosts.old  test_rsa