FB12543
Inch and a quarter OD, Type 43 material, the recommended HF BALUN Core Material.

Cat#   FB12543

FB12543  Size, 1.25 in. O.D (31.12mm). x .75 in. ID (19.05mm I.D). x .4 in. L (10.00mm L)

TYPE MATERIAL is 43  (µ = 850):

Type 43 is the recommended HF Balun "Core Material" and type 43 material
is optimal for the 1.0 MHz to 60 MHz shortwave (HF) spectrum.

For medium and  HF  BALUNS and wideband transformers up to 60 MHz.
Optimum frequency attenuation from 40 to 400 MHz.  High volume resistivity.
For toroidal cores, multi-aperture cores and special RFI suppression.