This glyph seems to be closely connected to the following glyph Aa7-13:.

Aa7-9 (509)	Aa7-10	Aa7-11	Aa7-12 (2 * 256)	Aa7-13 (3 * 171)

And we can in Tahua find another sequence of glyphs which at many points resembles the structure we are studying:

Aa3-66	Aa3-67	Aa3-68	Aa3-69	Aa3-70	Aa3-71	Aa3-72	Aa3-73
Aa3-74	Aa3-75	Aa3-76	Aa4-1	Aa4-2	Aa4-3	Aa4-4 (255)	Aa4-5
							Here it  then continues with a sequence we have looked at earlier.
Aa4-6	Aa4-7	Aa4-8 (259)	Aa4-9	Aa4-10	Aa4-11	Aa4-12

	*257
Aa4-4 (255)		Aa7-12 (512)

Furthermore, we can count from the beginning of the text on side a to glyph number 241:

	*225
Aa1-16		Aa3-66 (241)

Here we can guess an important meaning of 241 (as for example in the number of glyphs on the Phaistos Disc), i.e. it can be regarded as the sum of 225 and 16.

And then 257 will easily fall in line as the sum of 16 and 225 (August 13).

... sacred numbers were never chosen haphazardly ...