That should be a, not A. A will remove trailing 0x20 bytes.

S might be correct even on big-endian machines.

• Use S for native endianness.
• Use S< or v for little-endian.
• Use S> or n for big-endian.

If the OP was using a little-endian machine, we can only rule out the last one from the info provided.

See Mini-Tutorial: Formats for Packing and Unpacking Numbers for a convenient table of pack/unpack numeric formats.