5.5 u1 version check using api

I have 5.5 u1 installed. I'm trying to check the version of vcenter before register the html-bridge plugin.

When I make this call - mob/?moid=ServiceInstance&method=retrieveContent , I get version as 5.5.0 instead of 5.5.1. The build version is 1623101.

Is there is a reliable way for me to check if the vcenter version is 5.5 u1? I need this info for my code.

Thanks.