KSP won't let me load up a craft because it says I'm missing the following part, SSTU-SC-TANK-MFT-NFOT

I've messed around with the craft config file and found out that this is a semi-octagonal part that can be used as a fuel tank and a structural part.

I remember that this part has different visual model varieties like LF, LFO, O, and EC.

I have also reinstalled KSP and the SSTU mod a couple of times and cannot find this missing part. I'm sure the part exists, but I do not remember where I got it from.