Having translated a lot of Fraktur, I confidently suggest ‘I.J.G.’ is mis-transcription from the Fraktur script that was standard until 1941, I believe.
Although it’s very far from clear in the sans serif font this forum uses, that first letter was indeed a lower-case ell in the original typescript. Having struggled mightily with Fraktur in a few key paragraphs of “Die neue Gruppe”, I am hugely envious of anyone who can actually read the stuff.
Having an indicative date at which things would have changed is also very useful — I suspect that we see a lot more late-war than early-war sources, which would help to explain why I hadn’t met it before.
Upper-case ‘J’ was a recognised representation for upper-case ‘I’ in Fraktur, though publishers and writers varied in this use. The use of ‘J’ is illustrated in this map, taken from the German official history of the Battle of Mons. The likes of J.D. Nr. 18 is frequently translated as 18th Jaeger Division as another example. The ‘J’ refers to ‘Infanterie’, as in J.D. = Infanterie Division and J.R. = Infanterie Regiment:
Excellent, that makes perfect sense. I can sympathise with the people who make the “Jäger” mistake, as my first thought was to wonder if “Jägergeschütz” was a tribal alternative to “Infanteriegeschütz” — like British rifle regiments calling their bayonets “swords”.
Now I know about Js masquerading as Is before 1941, I expect I shall meet half a dozen examples over the next couple of weeks.
All the best,