First, a quick note about safety. Fred Sutter, W8QBW, was of retirement age in 1938. His designs suggest a fellow who had been working with radio for many years and like most of his peers, Mr. Sutter was more casual about high voltage than most hams are today; he assumed his readers know the risks.
He got away with it because he was very aware of the dangers. You probably haven't spent your ham career being "tickled" by the occasional contact with B+ batteries; you are unlikely to have ever used a #2 pencil to draw a hissing arc from the tank coil of a transmitter and it is almost certain you have never keyed a spark transmitter operating from wall-socket juice. So you and I don't have quite the same learned reflexes the old-timers had.
One of the less obvious "gotchas" in the QSL-40 is the DC power connection; it's a 5 or 6-pin socket on the back of the transmitter and the output from the power supply is connected to a plug. That's dangerous; there's all manner of juice on those pins. You could still buy that kind of plug and socket new through last year -- it looks as if Cooper Interconnect has discontinued them -- but they work just as well with the plug mounted to the transmitter side and the socket on the power supply.
The big RF coil's connections are "live," too; you must keep your fingers clear of them when tuning the transmitter. For that matter, the tuning knob provides the only isolation from HV DC on the shaft of the tuning condenser. He picked the right knob for the job, an Ohmite rheostat knob* made of real Bakelite with a deeply-recessed setscrew, but you probably haven't got one of those handy. So that may need to be designed around. The power supply design is simply unsafe; he used a ceramic-base knife switch to turn the B+ off and on, at a point 600 VAC above ground. He took precautions, but he had experience and parts that cannot be obtained today. I'm working on a "modernized" redesign of the power supply and hope to have a tentative version posted some time this weekend.
There's a little about safety. Now, about history--
There was a whole series of "QSL" rigs, from a 5W transformerless "portable" you could hold in one hand (though not when it was plugged in!) to a 100W push-pull design, with side ventures into a tiny receiver, making "air-wound" RF coils and the use of pilot lights as current indicators.
It had to start somewhere. It didn't start where you'd think or even with the goal you might expect:"Radio" was, in some ways, the West Coast competition to the ARRL's "QST." Formed by the merger of "R/9," a smaller ham magazine with very high production values and the original "Radio," a large-format pulp that evolved from a general-radio magazine to one intended for ham radio operators, it had its own style and tone, a little slicker than that of "QST" and focused on building equipment and operating -- mostly, chasing DX. ("Radio" eventually gave rise to today's "CQ" magazine.)
In his letter to the editor of "Radio," W8QBW is thinking about a 6L6 power oscillator merely as a driver for some big tubes, operating at several hundred Watts.
Instead, once he'd got his prototype working, he put it on the air -- and found himself on a very different path.
* Many models of rheostats had "live" shafts by design. So the knobs were designed to provide a level of protection the 1930s found adequate. Ohmite's design was ubiquitous and worked well; it's pretty much the default choice for hams and experimenters from the late 20s through WW II.