What you may be remembering might have been an ad similar to this one, from Boys' Life magazine, May 1970.
Note the text in the ad,
"Made of frangible zinc alloy, cannot be converted into an actual
Since the original weapons were not made of frangible zinc alloy, it is unlikely that the same equipment could be used to manufacture both the original and the replicas. If the replica did use parts made on the original equipment, then it would be strong enough to be used as a firearm, and could be modified just by 'fixing' the modification or replacing a couple of parts that made it unfireable.
A little research led to the fact that that original replica company, Replica Models,Inc. is still in business, now under the name Collector's Armory, Ltd..
Since we have a little more info from comments about the question, there is a category of replica weapons which might be relevant here, blank-firing replicas. A document here has information on 'illicit' conversions, focusing on three common types:
The alteration of a replica firearm to fire live ammunition was the most commonly recognized form of conversion.
The reactivation of a deactivated firearm is occasionally referred to as a conversion. A deactivated firearm is a genuine firearm which
has been rendered inoperable (i.e. incapable of expelling a
projectile). This form of conversion involves reversing the
deactivation process to enable the item to once again fire a
A semi-automatic firearm modified to have fully automatic (select-fire) firing capacity is the third alteration occasionally
referred to as a conversion.
Since the question seems to be focusing on this possible conversion process, the above source has a table detailing 27 various blank-firing replica weapons which can possibly be converted to firing weapons.
If the blank firing replica simulates fully-automatic fire, then theoretically some its parts might be usable to convert the 'action' of a 'real' semi-auto weapon into full-auto. The feasibility of this, and the source or originality of the internal parts used, would have to be addressed on a case-by-case basis. As you can see from the table in the pdf, and the retail page linked below, the list of replicas to individually evaluate would be quite extensive.
Incidentally, the retailer mentioned above, Collector's Armory, has a section selling blank guns, including semi-autos and fully automatic replicas.
These authentic Blank Guns are an ideal resource for training,
theatrical props, and re-enactment purposes. Many of these pieces are
the same prop guns used by filmmakers in Hollywood and the world over.