400 cubic inch small block crankshaft

I am trying to identify the crankshaft that came in a 400 small block engine. The casting numbers don't match in any book i have. the numbers are as follows:

8180
GM2n
3861528C or 3861628C the 5 number isn't perfect

Does anybody have any info on this crankshaft? I appreciate the help.

The only stock GM crank that ever came in 1970-1980 400 engines has casting number 3951529. It’s a one-off, i.e., 3.75” stroke with 2.65 main bearings and is externally balanced. You may have a replacement item?