Hello David, Hello Nick!
I do think so too.
This seems to be a sort of dialect form of "Posten", meaning sentry
the joke here seemingly comes from the fact that while the drunk gentleman is clinging to a lightmast, he sees this sentry on his post which looks like a hydrant and yells "Poshten" to differentiate the sentry from his own drunk behaviour...
but just a guess from me here too
cheers
phil