CloudSearch wildcard query not working with 2013 API after migration from 2011 API

I've recently upgraded a CloudSearch instance from the 2011 to the 2013 API. Both instances have a field called sid, which is a text field containing a two-letter code followed by some digits e.g. LC12345. With the 2011 API, if I run a search like this:

q=12345*&return-fields=sid,name,desc

...I get back 1 result, which is great. But the sid of the result is LC12345 and that's the way it was indexed. The number 12345 does not appear anywhere else in any of the resulting document fields. I don't understand why it works. I can only assume that this type of query is looking for any terms in any fields that even contain the number 12345.

The reason I'm asking is because this functionality is now broken when I query using the 2013 API. I need to use the structured query parser, but even a comparable wildcard query using the simple parser is not working e.g.

q.parser=simple&q=12345*&return=sid,name,desc

...returns nothing, although the document is definitely there i.e. if I query for LC12345* it finds the document.

If I could figure out how to get the simple query working like it was before, that would at least get me started on how to do the same with the structured syntax.