The 6LU8 and 6LR8 are essentially the same tube, the difference being a Novar (giant 9 pin) vs a Compactron (12 pin) pinout. As designed use of this tube was for a combination vertical oscillator and amplifier role in televisions. Physically in the picture you can see the separator and the 2 sections. On the left is the beefy pentode on the right is the Triode.
This tube was produced in quantities that make it good for experimentation. A way that you can possibly save cost on purchase is to buy one of the tubes designed for stringed filament service in 16V or 21V. That said even the 6V filament variants currently run well under $10 on Ebay.
The design maximum plate dissapation is 14 watts which is similar to the 6V6. Plans for “one tube” transmitters would likely be a good starting point for a design using these tubes. The Triode would end up being used for a crystal oscillator or buffer amplifier if one is in to more modern frequency control. There are smaller tubes based on a similar premise but they’re limited to tubes like the 6AF5 and variants.