Transferable MGs are investments...their value is only going to go up due to limited supply. Pre86, factory built and never fired is even more of a unicorn. If I had to guess the price on that bad-boy was probably in the $35-$40K range.
He stated that this is an all factory HK53 which to my knowledge there are ZERO transferable ones out there. Can't find the date they started making them but assuming after 1968. A couple may have slipped through the cracks like how some factory MP5s are supposedly out there but no one I know has admitted to owning one.
To be able to own a factory HK53 you must be a dealer or used to be a dealer. They are considerably cheaper then a converted HK53 on a Form 4 which a normal person could purchase. Still not "cheap" but definitely a lot more affordable.