Re: 11801C and the T1331 error: lost factory TB calibration data.
If you are out for exotic toys, try finding 1180x or CSA803x with fast calibrator.
You can tell it apart visually by the calibrator jack being gold 3.5mm type instead of Nickel plated SMA.
It's basically a TDR part of SD-24 in a separate enclosure used instead of regular calibrator.
They are rare but not unique. Here's random example https://www.artisantg.com/itemimages/Tektronix_CSA_803_View1.jpg
I have never found who and why did this and why this is not documented anywhere.