I have no idea what was used in '49 -- it's 10 years older than our trailer. That said, I'll offer a couple of semi-educated comments that will hopefully help.
It makes sense to me that they are talking about work-hardened rather than heat-treated aluminum. Anything with a T designation refers to temper, or heat treating. Different numbers refer to different levels of Temper, a balance between strength and lack of flexibility. I've worked with 6061-T6 and there is no way it would bend without cracking. Looking up 2024-T3, it is be less brittle, but not easy to bend. This alloy is prone to corroding if not coated (like Alclad).
Work hardening is very different from heat treating. To understand work hardening, take a scrap piece of copper and bend it back and forth a couple of times. Its surprising how much harder it is to bend after you work it back and forth a couple of times -- that is work hardening. Notice that it becomes more brittle as it gets stronger -- that is the strength versus brittleness trade-off. Keep that in mind as you try to make your curve. A lot of small corrections may be difficult as certain areas become much harder than adjacent areas as you work your curve.
If you search for recommendations on aluminum for c-channels, you'll see 5052-H32 recommended. That makes sense, given its workability and work-hardening characteristics. A thicker 5052 is where I'd start my search.