Kraft Promotion Dates (1968-1972)
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This article has been a long anticipated for myself, as I've been itching to comprehensively cover the Kraft sets in particular. Despite being basic assortments, they're very muddled in my mind and other sources are very confusing and even contradictory.
My goal here is to straighten all that out. That all starts with confirming the time line. How do we do that? Well, form numbers will be a big help, but our main anchor will be...
Velveeta Boxes!
The most well known source for the Kraft promotion was via boxes of Velveeta.
Regardless, first we will go over all the known Velveeta boxes and their relative appearances. I will be using newspapers to give relative dates(as they often show the Velveeta boxes) or other clippings that discuss the promotions.
Generally, given all the newspapers I've looked at and examples shown below, the promotion began or was consistently updated around September-October(as all reported orders seen below were in or around October), even though the boxes weren't updated in the marketing until ~February of the following year. All these newspapers have been tagged with "Kraft" and are available for reference in the site gallery.
The end of the promotion is a little hard to track, but it does not appear the promotion went very far into the next year if at all. Unfortunately, newspaper ads are limited in this regard due to reuse of images. However, there are other ways to find dates other than advertisements, which we do below.
And one more thing before we begin, I noticed that some states(Colorado, Oklahoma, Arkansas, Michigan, etc.) had no appearances of the promotion whatsoever. As such, the Kraft promotion may have been limited depending on certain states' laws or other limitations. The issue is that it's hard to be definitive about something like that, as ads may just not have been posted in those regions.
Regardless, the four phases:
1968
These first two years are almost the same, the first without the "Easy-Open Box". It's a little unclear when the change occurred, though newspapers were fairly consistent with their use of more recent boxes instead of reusing images over and over again.
The first Kraft promotion began sometime around October of 1968(known via this January 1969 newspaper). The LEGO Velveeta box itself first appears in an ad in early 1969.
To go along with this, the Kraft brochure H-2466's form number corroborates a late 1968 preparation:

1969
By February 1970, the box was updated with the new tear tabs and the image on the back was slightly altered. Even though it makes its first appearance in 1970, we can reasonably assume via the established pattern that the change happened in late 1969.
It does not appear a new form was developed for this wave.
1970
In late 1970, the promotion was completely overhauled with model sets instead of the 74-piece basic assortment. The box first appears in marketing in early February of 1971. The offer was evidently available as early as September, as this January 1971 newspaper reports they ordered one on October 3rd of 1970.
The Kraft brochure that came with the mailed set had a small change(removal of the carrying case), but otherwise was the same. Its H-5105 form number implies an early to mid-1970 compilation, as it is the number before the 1970 roadplate as well as other 1970 sets, which matches our late 1970 release.

1971
The "Mini Vehicle Sets" Velveeta box reappears in late 1971. This September 1971 newspaper is an excellent example, as it features the box in a unique grocery assortment image taken by the store and not by Kraft's marketing. It solidifies what I've seen across newspapers, that the promotion appeared late in the year. It'd make sense, as that's when to market a toy.
While the Velveeta box did not change, the promotion's brochure and offered sets were completely overhauled. The form number 8006 follows the established pattern, as it appears just after the 1971 contest blank for the 1972 Make a Model Contest and before the 248 Conveyor's instructions:

1972
The new box appeared in late 1972, evidenced by this early 1973 newspaper reporting that in November, they ordered two sets at $1 each. That'd only be possible with this updated box, as before the sets were $2 each:


The update to the promotion evidently would've required a complete change to the sets offered. The form number 8052 coincides with a mid to late-1972 compilation:

And because this is the most appropriate place I can think of to address this, I find it absolutely fascinating to think about the above paper in context with the ending of the LEGO license. Not only the wording "When you can't use your LEGO anymore...", but also the torn rainbow bars.
Probably reading into it a bit too much, but worth a note.
Also worthy of note, this last update in particular was an absolute mess that took several months to mop up. We discuss it further on its dedicated article.
Other
There may exist a fifth Velveeta box, though I cannot find any surviving examples. Typically I would ere on the side of this having existed, but an example should have surfaced by now. It may be a preproduction thing(as by October 1970 they should of had already released the next wave of sets) or could just be another, rarer variant.
It only appears once in late 1970:
