What type of engine in my MGA Coupe ?

[Saman P Perera <sperera@blkbox.COM>]

I recently was upset to find that my 1957 MGA coupe does not seem to
have the correct engine. The identification plate says that the engine
should be 15GB 25171 but the engine identification states that it is a
15F NH 20383. Does anyone know what type of engine this is and maybe how
or why people swapped them ?

Thanks in advance ...

Sam