I think we are only looking at events run to the F1 regulations of the time. You might have had a race for cars up to 5000cc and enter a F1 car that met that limit, but it wouldn't be a F1 race.
The regulations for Formule Internationale de Course No. 1 were, until about the Seventies, fairly broad. During the Seventies, they grew as more and more pages as safety requirements were added along with the usual bureaucratic desire to nail down specificity.
Also, keep in mind that these regulations pertained to the technical aspects of the machines.
From 1981 onward, the technical and sporting regulations were combined.
What is apparent is that many here still seem to conflate the technical regulations of the Formule International de Course No.1 with the sporting regulations governing the Championnat du Monde des Conducteurs that ended with the 1980 season into being one and the same.
It seems clear that the CSI and the FISA issued separate regulations fro each with changes noted as necessary for each season until the consolidation for 1981 (those regs for the NEW championship were issued in the October 1980 Bulletin, by the way).
Keep in mind that not until the 1961 regs for the Championnat du Monde des Conducteurs that the championship events were mandated to use the current F1.
That said, the Clowns in Blazers at the CSI/FIA did make it clear during the Seventies that if the various organizing clubs wished, they could include F2 and even "USAC" cars in their championship events -- a means to sidetrack the FICA/FOCA teams if necessary.
As pointed out, a LOT of events from the 1948 season when the current Formule Internationale de Course became the Formule Internationale de Course No.1 when the F2 went into effect.
For the 1952 and 1953 seasons, the organizing clubs could have run the championship using sports cars (or touring cars) if they so chose at the time. Or simply gone Formule Libre for that matter, as long as the ACN designated the nominated event as its "Classic Event" on the Sporting Calendar and its nominee for the championship.
If you are going to set some Terms of Reference or Rules of Engagement that reflect the contemporary/concurrent rules/regs in effect, at least try to make up your mind for the pre-1981 events whether you are looking at the cars (the formule) or the championship: apples and oranges.
Then, again, do what is done everyday, create your own Alternate Facts. Most people -- as it is apparent here -- wouldn't know the difference.