The 118A frequency generator is held to its special bracket with three machine screws. The bracket that fits into the 551B is quite different from the bracket that fits into the 551C. It is not merely a case of unscrewing/screwing. It's actually easier just to use a long tie-wrap or two to secure them, regardless of the KSU type. It's a moot point, since you will have more luck finding a Duesenberg in your neighbor's barn than finding the bracket. They are made of Evaporatum/Rapidium.

Arthur P. Bloom
"30 years of faithful service...15 years on hold"