If I create a symlink:
systemd@modem.service -> modem@.service

the unit - file fails badly: 

djunix:/usr/lib/systemd/system # ln -s mgetty@.service mgetty@modem.service
djunix:/usr/lib/systemd/system # la mgetty@modem.service
lrwxrwxrwx 1 root root 15 27. Feb 21:23 mgetty@modem.service -> mgetty@.service
djunix:/usr/lib/systemd/system # systemctl enable mgetty@modem.service 
Failed to execute operation: No such file or directory

the same with a hard link:
djunix:/usr/lib/systemd/system # ln mgetty@.service mgetty@modem.service
djunix:/usr/lib/systemd/system # systemctl enable mgetty@modem.service 
-> simply works.

I'd consider this a bug as the documentation states that "typically the unit
file is created as a symlink pointing to the @.service - File.

Thank you for looking into this,
take care



Dieter Jurzitza