Observations / Remarks
Let's begin to say that the (above) data is far too less to draw conclusions, let alone final conclusions, more data is needed but the current available data is good enough for an initial discussion.
1. Igel - The current data implies that the Igel NNUE (version 2.70 and 2.80) with a RMS above 100 and a SIM of 30-35 is dissimilar to the Stockfish nets. The webpage states those have their origin in Dietrich Kappe's Night Nurse network. With version 2.90 the author moved to an Igel 2.60 based NNUE. RMS=69 | SIM=57.
2. Fat Fritz 2 - is showing the strongest similarity of the engines we tested to date with Stockfish nets.
3. Minic - we learn from its website the author from Minic 2.46 to Minic 2.53 experimented with Stockfish NNUE but later moved to an own nets such as napping nexus and nascent nutrient which confirms our findings with the similar RMS to Stockfish.
4. Nemorino - the website states: Nemorino uses a slightly adapted file format compared to Stockfish, but you can use the netwok parameter files from Stockfish (and other NNUE engines) as well. Those files will be copied and converted automatically at first usage by Nemorino. It's hard to comprehend what this means in practice and so we ran Nemorino with a SF13 NNUE. The result (like the evaluations) was unrealistic (MSR=156.18 and SIM=21.19), also no copy was made.
5. Rubichess - From the website we learn the author created its own NNUE. The RMS implies that is true.
6. Orion 0.7 - An experimental NNUE version using a Stockfish net as the RMS shows.
7. Orion 0.8 - The author states the NNUE is an original work and seems to confirm our findings.