iptables is changing IP addresses that start with 047. to 39.!

This is what is happening:

$ printf "%d\n" 047
39

047 in octal is 39 in decimal.

You just need to drop the leading 0.

At a guess, this is happening because something in iptables is splitting IPv4 addresses into 4 decimal numbers so it can convert the IP string representation to a long. But that's conjecture.

inet_aton also accepts a couple of other less usual forms (the manual actually even describes them):

octal:
020.0.1.22     ->  16.0.1.22
hexadecimal: 
0x10.0.1.22    ->  16.0.1.22
combination:
020.0.1.0x16   ->  16.0.1.22
bottom two bytes together (old Class B)
16.0.278       ->  16.0.1.22
bottom three bytes together (old Class A)
16.278         ->  16.0.1.22
all in one, hex
0x10000116     ->  16.0.1.22
all in one, decimal (completely unreadable)
268435734      ->  16.0.1.22
this should be simple
0020.0426      ->  ...

They're likely to work on web browsers too.

Prefixing octal numbers with a zero, and hexadecimal numbers with 0x is at least as old as the C language.