WAP; I had intended looking up that 21 joint, as i was recalling it was indeed the DTCL, but as you have shown & Rabbit conceded saved me the trouble.
If you look at #4 DT joint, & imagine extending the flats marked A all the way to the lower bbl flats, eliminating the steps B, then you have what I referred to as "Conventional". You will note there is then no mechanical strength, only that of the brazing, to hold the lump against the bbls.
#5 also depends soley on the brazing but has a much larger area, but tends to more depth of the breech. This is the joint I referred to as a "Shoe Lump". I have a ca 1900 J P Sauer & Son 12ga back action sidelock with this shoe lump breech.
A picture is worth a 1,000 words, indeed thank you Ian.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra