I've come to the same conclusion, I can find the arm but struggling to find the heavy duty bracket, only one I did find was in Italy and shipping cost more than the bracket
Ergotron HX arrived yesterday so I took the plunge to install and fit. A bear of a job with a 49". The instruction's state twist right to tighten the thing and it flops all over the shop. Anyway having burst a vessel to get it installed, the ballache is worth it; It is expensive! I had initial buyers remorse but really pleased with the desk space recovered.
My desk is not that deep (Kitchen Worktop 25 inch/63.5 cm depth) using the standard tripod stand encroached the useable space of the desk. Additional height is very welcome and looks much neater.