It turns out that iptables doesn't handle leading zeros too well. As $machinenumber that is used has to have a leading zero in it for other purposes, the idea is simply to create a new variable ($nozero) based on $machinenumber, where leading zeros are stripped away.
$machinenumber is a two-digit number between 01 and 24. Currently it's 09
$machinetype is 74 for now and hasn't caused any problems before.
What I have so far is:
nozero = (echo $machinenumber | sed 's/^0*//')
iptables -t nat -I POSTROUTING -s 10.($machinetype).($nozero).0/24 -j MASQUERADE
While I believe I'm on the right track, the code results in:
ERROR - Unknown string operation
you can also do
machinenumber=$(expr $machinenumber + 0)