Replacement under frames and floor for AHM ho boxcars?

[modelsof1900]

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I would like to finding a suitable fish belly frame for the Helium cars. Hope this is useful - Dave

I'm surprised to read of fishbelly frames for helium cars???
Compare this foto - https://www.flickr.com/photos/mikedannem...13183@N21/ or see some more pictures of this flicker group - https://www.flickr.com/groups/2713183@N21/pool/.
There air very ordininary straight frames on the helium cars however they have a very strong brake equipment where all components exist in a double installation. I have a good picture from a well detailed O-scale model of such cars however I must find it yet.