The MGB rack in my Unique recently failed the same way Motorhead's did. My car went 5600 miles on the first rack.
While researching whether to rebuild my rack, or just replace it, I found these discussions talking about the bushings on an MG forum:
Steering Rack Service : How-To Library : The MG Experience
Steering Rack Bushing Replacement - solved : MGB & GT Forum : MG Experience Forums : The MG Experience
Inside the boot on mine, the RH side looked very much like the photos Motorhead has posted. Nothing but grease and grit- no sign of any bushing remnants. I did notice that my RH boot was torn inside one of the grooves, and the MG guys all said that it does not take very much road grime inside there to chew those soft bronze bushings up real good....
So, I went ahead and ordered one of the Moss/Argentinian replacement racks for my car, and since all the other components are still good on the rack I removed, I plan to order new boots and rebuild it, so that if the new one also fails prematurely, I'll have a standby replacement ready for service.
But, I may forgo the bronze bushing on the rebuild, and might make my own bushing out of PTFE instead, and then fill the rack with Silicone grease instead of petroleum grease....