Hey JOTD <3 There seems to be a difference to the arcade version: you are not guaranteed a shield drop before the dragon on level 5, and it's the only weapon that can defeat it.
it's completely random. On the arcade version you can have the shield on level 5 or even on level 4 but it's not guaranteed either. Sometimes the bonus can be lost, or worse the game can give you ... the stupid axe. The dragon can be defeated by anything. The shield is required for the 2 satans on level 6, and for the final boss of course. Disclaimer: I never could get past the 2 satans with the shield without cheating.