I have little doubt they were outsourced. I call them "factory" because they were offered by the factory as an option, no matter who was the actual manufacturer.
If you can get your hands on the December 1989 issue of Guns&Ammo, you will see these grips on serial number 150918, an early P5c. The author of the article stated he thought the grips are made of beech wood.
All subsequent photos I've seen of the P5c in factory trim have had the plastic grip panels. The photo I posted was something I snagged off an auction site recently. Until I saw this photo I thought the wood grips on the P5c in the G&A article were a fluke or a one-off. But, given the above photo, it appears that some production pieces left Walther wearing these grips.