FK 15 = Mondragon system, 7x57mm, a/c modified takes drum mag. Also used by KM.
FK 16 = Mauser system, 7,9x57 (lubricated)
Both types definitely used by GAF as backups to MGs on some fixed-wing and lighter than air a/c (Zeppelins); probably more M15s than M16s (the latter extremely expensive and slow to produce).
M15 in two versions: adapted for drum (a/c) and not adapted for drum (a/c, also infantry and navy vessels*). M15s relatively common.
M16 in three versions: a/c (relatively common), infantry (not common), fortress (series production dubious).
*Possible exception: KM Zeps?