Based on a lengthy investigation, here is what we know so far:
Field elements: 254-bit number, no flags
Group 1 elements: two 254-bit number, two flags on the last byte. The MSB in the last byte is the Y is negative flag, and the next one is point is infinity. it is not yet clear if the last to bits on the first number are always 0.
Group 2 elements: same as group 1, but has 4 numbers
Based on a lengthy investigation, here is what we know so far:
Y is negativeflag, and the next one ispoint is infinity. it is not yet clear if the last to bits on the first number are always 0.