The sack must really have gone into hiding after 1930. You don't see the word used anymore, and almost all jackets are darted and hourglassed. Being a well-upholstered fat cat wasn't something a man wanted to flaunt!
The "Full Box Effect" might have worked in 1903 with a high bowler hat, but in 1928, ouch!
About the Ulster coats, here's a definition of the style: 6x3 DB front, stout protective collar, thick welt, patch-flap pockets (No. 726 has only the flap), cuffed sleeves, and tweed or similar heavy goods. Puristically, real Donegal tweed (I'm sure these weren't).