the use of the barlow seems universal to stereo ,double tubed scopes. such lenses will cut the optical power in half. its' use will also double the working distance. in ordinary engraving, the resulting cut in magnification will not likely be problematic, as you will have plenty of power for most if not all normal engraving. the use of occulars of higher powers would not alter the working distance.
Keep checking E-bay, I got an original NOS Unitron .5 Barlow for $ 9.95 a while back,( right place at the right time). They are the answer for working space under the scope. Good luck in your search.
Cheers
Don
this may not work, or even apply to your situation.. i bought a leica on ebay. it came without a .5. none available at the time on ebay either. i took a chance, and bought a different.5. the thread diameter was about 2mm less than the leica threads. even so, it centered very nicely. i simply used hotmelt glue to affix the lens to the scope body. in doing so, i got 71/2" of working distance, but has worked fine for several months. it's possible the leica may have only delivered 71/2" of clearance with a leica .5.