I simply said going by the test in my thread, it could be common mod for headbutt and target for cassis.
Situational evidence due to the values all being very close together and an effective damage range of more than 10k. With enough elemental/enemy size/enemy type/etc stats, it could easily change the math entirely.
Having tested more with @DontMindMe, these are done with no damage range (Min attack = max attack), and no extra stats:
-Rim Blow Level 6 = 262
-Rim Blow Enhance level 50
-Attack = 1106 (high guard included)
-Leather Enemy Defense = 214
Damage dealt with High Guard active = 5279
((1106 + 262) - 214) * (100% + 230% - 25%) * 1.5 = 5279.55
Damage dealt with High Guard and Cassis Crista = 10074
Target? ((1106 + 262) - 214) * (100% + 230% +400% - 25%) * 1.5 = 12203.55
Common? ((1106 + 262) - 214) * (100% + 400%) * (100% + 230% - 25%) * 1.5 = 26397.75
Base? ((1106 + 262) * 4 - 214) * (100% + 230% - 25%) * 1.5 = 24055.35
All too high. So we go backwards instead :
10074 / 1.5 / 3.05 + 214 - 262 - 1106 = 1047.967…
262 * 4 = 1048
So Cassis is [Skill Attack * (100% + 400%)] at level 5 attribute for Peltasta skills. I already added a new section to my post specifically for these type of modifiers (directly changing Skill Attack) since I know Effigy works the same, it was just the only one known one before.
As a bonus, we did Headbutt as well after (The attribute in this test was level 4, so 40% additional strike damage):
Damage dealt with High Guard + Cassis + Headbutt = 11395
((1106 + (262 * (100% + 400%))) - 214) * (100% + 230% + 40% - 25%) * 1.5 = 11395.35
Considering a common would be
((1106 + (262 * (100% + 400%))) - 214) * 1.4 * (100% + 230% - 25%) * 1.5 = 14103.81
Headbutt is without a doubt a Target modifier. And Cassis Crista, at least for Peltasta skills, is a Skill Attack modifier.