Sets/Variants That Never Existed
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The following is a list of Samsonite sets and box variants that never reached production. Many of these are commonly cited to have been available at normal retail, but no evidence of their release nor surviving examples have surfaced.
As a note, this list will largely omit known prototypes, as those were never meant for mass retail. The exceptions are ones that have been widely misidentified as having released or are historically significant enough to warrant discussion.
With that out of the way, let's begin.
301 Wheel Toy
While this was a real set released in 1968, the image commonly attributed to it is not. Linked below is the image used in aggregate catalogs(this one is from BrickLink at the time of writing).
It is very clearly a prototype of the 1605 Wheel Toy Canister being misattributed to the 301 set, but we needn't rely on conjecture as we have the source of the image, from a 1963 Spiegel catalog clearly showing it is indeed a 1605:

The actual 301 set came in two different versions. One was in the blue Samsonite attache case and was pictured in the 1968 Sears catalog, seen in the lower left corner:

And for 1966-67(also through Sears), came in a plain box:
110 & 111 Accessory Packs
These set numbers were given in a Samsonite pack-in catalog from 1972, 110 being the Vehicle Accessory Pack and 111 being the Building Accessory Pack. By 1972, Samsonite knew they were losing their license and were busy liquidating their inventory:

These sets were already being produced and existed by 1971(identifiable here), but existed under different numbers. The 166 Vehicle Accessory Pack and the 167 Building Accessory Pack, while having different numbers, are found to have the exact same piece count as the 110 and 111(as well as weight, as listed in the 1972 Dealer Catalog).
Why would they change the numbers, just to revert back to the sets they already had in production? No one knows, but given Samsonite were already losing their LEGO license in the US, it is highly unlikely that two versions of the same set were produced. Instead, it'd be easier to use what already existed. That does, however, make the absence of the boxes being pictured here highly unusual. It could be because they were supplements and could not independently build any models.
717 Junior Constructor
This one is more known because everyone knows it was never produced, but I thought it worth including due to its infamy and the developing discussion surrounding it.
The prototype model featured prominently in advertisements starting in 1961, even appearing in a few newspapers from 1961:

Here is the same image of the 717's prototype box:

It's a much, much larger model than the released one, likely too large for the finished box. It may be a matter of perspective, as this is the only image that I'm aware of featuring it, but the prototype box seems larger than the released box.
It's unknown why Samsonite would've even considered making a set such as this, as it would've been far too expensive to be a reasonable set for an average household. As I've thought about it, my theory is that it was never intended for release, at least not that model specifically. Instead, because that house model existed as a retailer model prior to 1961(in Europe), and considering everything else here is a placeholder, it was likely just a placeholder that was used in finalized materials without much thought to the impression it made.
Interestingly, contrary to that, 717 sets typically include a number of 1x6 bricks in blue, when the modern home model doesn't use them. The prototype home, however, needs them for the chimney. It's a subtle gesture, one that may suggest the mock up model may have actually been intended for sale at one point, even in a reduced capacity.
More information can be found on the 717's dedicated page.