What do the letter on the bottom of the block mean?

[itx]

I've got an 88 T1 I pulled apart and I was curious as to what the letters, one next to each cylinder on the bottom of the block mean.... Starting from cylinder one it goes C, D, D, C

[mo' parts]

piston size. they correspond to slightly diff. size stock pistons

[black86glhs]

Ding,ding...we have a winner!

[itx]

Well then good thing I marked the pistons when I pulled them

Also, I noticed the rods all have to same part number, but the lettering is different sizes between 1,3 and 2,4... Almost makes me wonder if someone else has had to replace a couple before I got it...