A large ferrite toroid as used in our QRO CobWeb balun (2 ferrites are required if you're making your own) and also used as part of G8JNJ's UnUn tests. This ferrite is not suitable for general-purpose baluns/chokes across HF - use a 31-mix or 43-mix.
Yes, these are quite pricey compared to their 31+43-mix cousins!
Toroid sizes/types are easy to ID, the "240" translates to 2.4" (diameter) and the "61" is the ferrite mixture. What type do you need? 61 is used for the G3TXQ CobWebb balun, 43 is good from 7MHz and up (and a good general-purpose 7-30MHz solution) while 31 is good for lower HF bands. Remember, you can stack the cores for increased choking and power-handling. As always, ratings need to factor in the aerial (SWR) match and duty-cycle.