Garry, I looked it up. The BBC miniseries I was referring to is called "Maps: Power, Plunder and Possession", and tells us about a number of political aspects in addition to the history of map making. I found a brief description here:
http://docuwiki.net/index.php?title=Map ... PossessionBack to Yugoslavia:
The "Greek Datum" turns out to be a problem. The simplistic transformation I found that just adds a few arc seconds to the more recent GGRS87 datum might work for areas near the origin in Athens (which I haven't tested) but is pretty useless for Yugoslavia. I tried a map sheet for the Danube, near the Hungarian border, and the error was about 500m!
What did succeed, however, was a test for the Lambert projection. I tried the "Red" grid (the "Danube" zone as defined by the War Office). And the red Lambert grid matched the geographic coordinates (blue longitudes, Greenwich prime meridian) quite well.
But without a proper transformation into the WGS84 world, no DEM will fit.
What I can try is to compute a custom Helmert transformation. It needs a series of matching coordinate pairs, their world vs. our world, scattered across that entire region. These coordinate pairs would then be fed into some external geodesy software that solves the linear equations and delivers the 7 Helmert parameters (3 x translation, 3 x rotation, 1 x scale), applying regression and minimum mean squared error.