Nitpick: The terminology used by IBM on the 3270 family (including the 3277 whose keyboard you shared) was "Tab" and "Back tab", not "Next field" and "Previous field".

▲ robocat 5 hours ago | parent [-]

Yes: Page 30 of The Operator's Guide for IBM 3270 Information Display Systems calls it a Tab key.

   Tab also has typamatic capability that allows you to move the cursor quickly from field to field.

  The Back Tab key moves the cursor back to the first character position in an input field. If the cursor is already in the first character position of an input field, and if you press the Back Tab key, the cursor will then move back to the first character position of the preceding input field.

So my guess is that the cursor defaults to being at the start of a field as you navigate, so Tab and Back Tab work as expected. But if you're editing a field and have moved the cursor within the field then Back Tab acts differently.

https://usermanual.wiki/Document/GA2727421OperatorsGuideforI...

	▲	jasomill 3 hours ago \| parent [-]
		This is correct. It's also worth pointing out that the 3270 defaults to overwrite rather than insert mode, and has an "Erase EOF" key that deletes all text from the cursor position to end-of-field.