A technical appearance of sorts: You Are Spider-Man Vs. The Incredible Hulk.

It's another choose-your-own-adventure novel and it has a few chapters at the start that include 'Dr Walter Langkowski... one of the world's top experts on radiation..."
There are no illustrations of him however.

The OHOTMU HC adds it as continuity so if so it's after the 'birth' of Spider-Man & Hulk, but before Walt became Sasquatch.