[**Onpass**](http://en.wiktionary.org/wiki/onpass) does come up in dictionaries as a valid word.

But, its usage is very rare.  [NGRAM](https://books.google.com/ngrams/graph?content=onpass%2C+pass+on%2C+on-pass&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Conpass%3B%2Cc0%3B.t1%3B%2Cpass%20on%3B%2Cc0) shows it to be nearly nil.

I don't think there is any reason to use one vs. the other.  The meanings are identical.

If a particular office is using it, it likely is a local (to that office) cultural expression.