My guess is the AVR400 is for the 815B and 816B 400Hz units, but that's just a guess. Guyfang can probably tell us for sure.
Your guess is right!! NOW, keep in mind, there is a Parts manual that shows them backwards. The TM 9-6115-672-24P manual has a mistake about these parts. Disregard this manual when it come to control panel parts. Use the 30 KW manual.
Here is an easy way to tell what is what. The info below was taken from the parts manual TM 9-6115-671-24P, (30 KW manual) figure 5, item 61. Page, (PDF page number) 58.
61 PAOZZ PAOOO PAOZZ 30554 96-23542 REGULATOR,VOLTAGE.....................1
UOC: LTY
61 PAOZZ PAOOO PAOZZ 0BXW5 AVR100 REGULATOR,VOLTAGE.....................1
UOC: LTX
The highlighted info is the UOC. Usable on code. It tells you what set, 400 hertz or 60 hertz set the part goes in.
Un-coded items,
(no UOC listed in the book, next to the part) are applicable to all models. Identification of the UOCs used in the RPSTL, (parts manual) are:
Code Used On
LTX Generator Set, Skid Mounted, Tactical Quiet, 30 KW, 50/60 Hz
LTY Generator Set, Skid Mounted, Tactical Quiet, 30 KW, 400 Hz
This info is listed in every parts manual in the Army. I found this info listed in the parts TM 9-6115-671-24P, page (PDF page number) 19
Superman,
NO, you can not use them interchangeably. There is several other differences between the 60 hertz and 400 hertz machines, besides the switch in the control panel.