870 Rem. SPS barrel question?
 

I am shopping for a used 28" or 30" Rem-choke barrel and have located a couple. The problem is that in both cases the sellers want to sell me a barrel made for a Remington Express 870. I think I remember reading at one time that Express barrels are not interchangable with the Special Purpose or Wingmaster 870. Does anyone know for sure? Thanks.