To contain the acid, use wax to build a dam around the boss. Try alcohol first (low surface tension) to level the case. Dilute some acid with water for a test. Apply acid with a Q-tip in several passes. If it bubbles immediately it's strong enough, if not use less water.
When you see something appear, stop the acid with baking soda and rinse. Scuff the surface with Scotch-Brite etc. to see what you have. If too faint, repeat.
This will not work if the numbers have been completely ground off to roughly 2.5 × the impression depth (.150"?), starred or X'ed over, or welded.