currency is a native field.. it should default to whatever is on the customer you've making the sales order for

Yes of course. The point I'm raising is that it doesn't specify the currency type on the transaction to allow the user to know what they are looking at off-hand.

the currency field is usually under the Accounting tab of the sales order. Seems like the form is customized to hide this tab

Are you asking if its possible to display the currency symbol (e.g. $) in the summary box?