C function argument types

Hello

How can I determine the pg_class oid of the argument type in a
polymorphic C function when it's called with a table row?

PG_FUNCTION_INFO_V1(myfunc);
Datum myfunc(PG_FUNCTION_ARGS)
{
Oid arg_type = get_fn_expr_argtype(fcinfo->flinfo, 0);
elog(ERROR, "arg_type %d", arg_type);
PG_RETURN_NULL();
}

select myfunc(mytable) from mytable;
arg_type 65754

select 'mytable'::regclass::oid
65752

I expected them to be equal.
What gives?

For other table the results are 65783 vs 65785.

I'm running v 8.4.3

Thanks,
Bogdan