Yanagisawa's PIC viewers supported multi-part images, split amongst many files.
For that, they recognize images that have the same name, but differ from the last letter, that also encodes the position of that image in a grid.
Two naming conventions were used
1) Bottom (D or S) + Up (U). Always 2 images, arranged as a single column. Example grids:
U
D
--
U
S
2) Starting with A. All images will be placed vertically in two columns filled top-down, each with half of the number of files. Example grids:
AB
--
AC
BD
--
AD
BE
CF
--
AE
BF
CG
DH
And so on...
I'm attaching here many samples of such multi-part images, for testing.