Hi again Bob
If you just want to add a letter or a number to your parts, "and I`m guessing they would be for reference right"?
The easiest way to do this would be to set a new layer and on this layer draw the letters/numbers where you want them and send this layer to the printer and run the job as a Vector cut job - yes cut job! All you need to do is make sure the lineweight for this layer is again 0.01 or similar but this time the settings are:
*Speed = 100 %
*Power = 1-5% (you will need a low power)
*Frequency = 500% or whatever the laser`s recommended settings are!
Running at these types of settings in a cut (not engrave "Rastor") mode will run the job through at top speed and will achieve a "score/etch" cut and will not cut through your material thus giving you a lightly scored reference detail.
I do this constantly when I need to achieve reference lines, contours, details etc. for architectural model-making. Beleive me this will be the most efficient and fastest way to do it because engraving will make the laser carriage travel across the bed like a standard paper printer and take a lot longer.
Hope this helps.