I don't know why it's not in TCR. Check with the authors as they may have definitive info. Or whoever has the Small Key mold order cards (if they exist?) may be able to look it up. Here's some dates & postcards to consider.

Below are 3 postcards, all from the MoGH ChipGuide. All 3 show the Holiday Inn hotel-tower in the background, all at the same mid-Strip location. In those days, I believe the hotel was run separately from the casino below. In fact the gaming license was likely NOT in the Holiday Inn hotel operator's name (corporate Holiday Inn or a franchisee?) but in the name of a local casino operator.

Note in the first postcard, there's an image above the paddlewheel shaped entrance on the side (in red) that I think says "River Queen".
In the second postcard, a later image in which the name above the entrance has been changed to "Holiday Casino" (but from the Holiday Queen era?)
In the third postcard, the original riverboat (the bow or nose of the boat pointed perpendicular into the LV Strip) has been torn down and the new "Holiday Casino" riverboat has been turned sideways ("sailing" parallel to the LV Strip).
And of course, quite some time later, Harrah's took over the property and changed the facade even further over the years.
I was in the building as far back as when the riverboat "sailed" perpendicular to the LV Strip, but I didn't start collecting that far back, so I have no first-hand knowledge of chips from that time period.

The reason I point out this evolution is that the (separate) casino had a pretty small footprint, including a few live games plus slots, but not huge. In the ChipGuide, the River Queen is dated as 1972 to 1973, then the Holiday Queen at 1973 to 1974, then the Holiday Casino from 1973 on. A fluid period in the 1972-74 range. In fact, the rare Holiday Queen chips are listed as made/issued in 1972, prior to its name change in 1973. And the Holiday Casino chips with the HOUSE mold and riverboat logo are shown as made/issued in 1977, though we also know of their $1 Bicentennial chip issued in 1976. So the 1972 to 1976 period had several types / names of chips issued in that Holiday xxx building along the mid-Las Vegas strip.

So what? ... Consider that the Holiday Queen chips ($1 and $5 and larger denominations) likely were the first ones used on their blackjack tables (maybe just a $2 minimum back then?) but to pay off a $5 blackjack, you needed a $5 plus 2 x $1's plus a half-dollar silver. Perhaps (and this is conjecture) the pit boss said let's get a box of $2.50 chips instead, but for a small single order box, it was easier or faster to get them in the small-key hot stamp mold.

BUT --- why the "River Queen" name instead of the "Holiday Queen" name like the other chips? That question I can't answer. If I am reading the first postcard correctly, as "River Queen" and on an artist's rendition of the project, maybe they were considering the "River Queen" name from the start, then changed it to associate it with the the Holiday Inn that's behind it? That's my best shot at a plausible explanation. Don Leuders, in his time, was a reputable and fairly high-end collector, so it has some provenance there. That's all I got!