The engine ID is stamped on the flat horizontal pad at the front of the engine.
For the 300E the engine ID begins with the letters "MR".
On AC cars, the compressor sits above the ID pad and the ID is nearly impossible to see.
Many engines will appear to be stamped "MP" but a close look
should reveal the faint leg of the "R"
You can also see that the top of the line with the "MP 4I3" characters is deeper and bolder,
more legible than the bottom of the same line.
When the engine was stamped, the ID tool was not square to the surface.
Thanks to Soren Persson in Sweden for these photos.