I have a drawing (.dwg) file that consists of one model with two layout tabs. One layout tab is an assembly drawing (to the client), the other layout tab is the detail drawing (to fabrication shop). We create drawings in this manner using layers that are turned on or off in the layout viewports to be show graphics that might be needed in the detail drawing, but not in the assembly drawing and vise versa. The reason we create drawings this way is so if something changes, we change the model and it changes in both the assembly drawing and detail drawing. Each Layout tab has a unique number as follows:

I didn't have any part of the drawing number convention. Company had it in place when I was hired. I would have done something easier.

It makes sense to have only one model drawing as it simplifies revisions.  If you split them you'd quickly run into all sorts of trouble trying to figure out which file has the revisions you want.  But when it comes time to keep track of whether a print is for a client or the fabrication shop you do need another system.

Have you thought about using the layout tabs themselves to help with print file naming?  I like to use the system variable "CTAB" as my Layout name.  This is then called somewhere in the title block and gets used whenever I print that sheet to either a PDF or DWF.  If you go with John's idea of appending a "D" or "A" to that name you'll always have a set of print files that will easily identify themselves.

As for the model filename itself... isn't that always going to be the base part number?  If so, simply save those files with nothing appended to them.  Let the separation happen with printing.  This way you can continue to use multiple layouts as you have in the past.

Dave.