Questions REST Web Services Filtering

Hi Tim and Forum Users,

I am trying to extract invoices for further processing using the Contract REST Web Service endpoints.  But, I only want to extract and process invoices that are related to a sales order document.  Within Acumatica (full disclosure, I am a novice), I can filter using "OrigModule" and choose "SO" and only see those invoices that came by way of a sales order.

But, I cannot seem to figure out how to filter by that using the $filter parameter of the web service and I also do not see that value from the invoice actually exported out as part of the JSON result set when you export it via the REST web service.

So, I am wondering if I'm maybe not able to filter by that and also it seems that maybe only a smaller subset of the invoice data is exported out from the web service.

If you could help shed light on either/both of these, it would be much appreciated.

Thanks!