I bought my Dapol knuckles several years ago, just in case I might need them someday, but I never actually tested them. I just dug them out, and I didn't even realize I had 2 different versions. One is a simple 1-piece casting, similar to the dummy Red Caboose couplers. The other one is similar in appearance to McHenry or Bachmann coupler where the knuckle opens up and has an external spring. Similar to what is shown in Mark's link. Sort of like an attempt to make the coupler magnetically operated (since the NEM 355 pockets either do not swing sideways or are too stiff to swing just from the force of the magnetically operated trip pin).
The bottom line is that I just tried mine and Mark is correct. They are too low to work. Not sure why Dapol produced those in the first place, since they are useless for coupling with standard MTL couplers. They could have probably made them using offset shank (like MTL does on some of their couplers) to match the standard MTL height.