/usr/share/calc/help/custom_cal is in apcalc-common 2.12.4.4-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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The following custom calc resource files are provided because they serve
as examples of how use the custom interface.  The custom interface
allows for machine dependent and/or non-portable code to be added as
builtins to the calc program.  A few example custom functions and
resource files are shipped with calc to provide you with examples.
By default, the custom builtin returns an error.  Calc have been
built with:
	ALLOW_CUSTOM= -DCUSTOM
in the top level Makefile (this is the shipped default) and calc
must be invoked with a -C argument:
	calc -C
when it is run.
See the ../cal/README or "help resource" for information about
calc resource standards and guidelines.
=-=
argv.cal
    argv(var, ...)
    print information about various args
halflen.cal
    halflen(num)
    Calculate the length of a numeric value in HALF's.
pzasusb8.cal
    Run custom("pzasusb8") on a standard set of data, print Endian
    related information and print value size information.
## Copyright (C) 1999  Landon Curt Noll
##
## Calc is open software; you can redistribute it and/or modify it under
## the terms of the version 2.1 of the GNU Lesser General Public License
## as published by the Free Software Foundation.
##
## Calc is distributed in the hope that it will be useful, but WITHOUT
## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
## or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU Lesser General
## Public License for more details.
##
## A copy of version 2.1 of the GNU Lesser General Public License is
## distributed with calc under the filename COPYING-LGPL.  You should have
## received a copy with calc; if not, write to Free Software Foundation, Inc.
## 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
##
## @(#) $Revision: 30.1 $
## @(#) $Id: CUSTOM_CAL,v 30.1 2007/03/16 11:10:04 chongo Exp $
## @(#) $Source: /usr/local/src/cmd/calc/custom/RCS/CUSTOM_CAL,v $
##
## Under source code control:	1997/03/08 20:51:32
## File existed as early as:	1997
##
## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
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