Fairly large (They are 5mm longer than 32650) LiFePO4 batteries that can deliver a lot of current.
The two batteries matches very well. Being LiFePO4 the discharge curve is very flat, until the battery is nearly empty. The batteries can also easily handle the 30A test current and without any drop in capacity.
The charger curve for LiFePO4 is significantly different from LiIon.
Conclusion
Need a lot of current and stable voltage at around 3V, then these batteries are very good.
LiFePO4 are not very good at capacity vs. size/weight, but can usually handles many more charge cycles than 4.2V LiIon.
This makes them very useful in application where you charge/discharge very frequently.
The voltage also fits nicely with 3.3V chips, i.e. a small LiFePO4 cell and a solar cell would work nicely for a low power application (I am also testing some small LiFePO4 cells).
I replaced a 6V lead acid battery with 2 LiFePo4 cells (2 in series) in a solar light. The original lead acid battery did last about one year (one hot summer probably killed the lead-acid battery).
The LiFePo4 cells are still working perfect for more than 18 months now (and these survived the hot summer of 2019).
Because of the voltage these cells are a very good replacement for 6V - 12V - 24V lead acid batteries, the working voltage of 2 LiFePo4 cells in series is about the same as a 6V lead-acid battery.
Just use a decent battery management system to protect the cells from deep discharging and overcharging and they will last a lot longer than lead acid batteries in applications with many charge/discharge cycles.
Because of the very high current that these cells can deliver it is best to use some kind of short circuit protection as well in a LiFePo4 battery pack.(most BMS have short circuit protection)
LiFePo4 is also much safer than lithium-ion, they don’t burn or explode as easy as lithium-ion.
The only drawback is a lower power density of LiFePo4 compared to lithium-ion cells.