It most certainly has a cast iron frame. I can't imagine it withstanding any amount of tension without one.
My advice: stop now before you destroy it.
Don't be so sure. It's a "140 years old vertical french piano." Wouldn't surprise me at all if it didn't have an iron frame.
"Antique" importers brought thousands of old pianos into the U.S. during the 1970s and 1980s. Most of these were of English manufacture, but not all. I've also seen a few old German and French pianos over the years.
The main criteria for these pianos was that they have interesting, often quite beautiful, cabinetry. The people bringing them in had no knowledge of, or concern with, the interior works. The structural condition of the pianos did not matter. Nor did the condition of their actions, strings, pinblocks, etc. As long as they looked pretty they could be -- and were -- sold at a profit to gullible Americans.
Many, if not most, of these things were structurally deficient in several ways. Their designs were often, at best, marginal. Their transitional actions, while historically interesting (and sometimes significant) often had rather poor performance potential regardless the amount of time and effort poured into them.
You are quite right about its ability -- or lack thereof -- to withstand much tension. It is unlikely that any overstrung upright piano strung with anything remotely resembling a standard ("modern") stringing scale but lacking at least the rudiments of a metal frame is going to be capable of any long-term stability.
ddf
