extra dns zone files for backup zone?

Is it normal to see files like this:

-rw-r--r-- 1 root wheel 2189 Sep 19 17:37 zone.org.bak
-rw-r--r-- 1 root wheel 0 May 1 04:23 zone.org.bak.6dKaRF
-rw-r--r-- 1 root wheel 0 Sep 17 20:22 zone.org.bak.8u8Y1n
-rw-r--r-- 1 root wheel 0 Aug 19 17:39 zone.org.bak.ZxKtb7

....for a backup slave zone? These are sitting on a backup ns server that I
haven't examined in a little while.

Thanks,
Frank