What a detailed explanation you have provided! Thank you for taking time to answer my question!There are actually two ways of forming causatives.
In Classical Japanese, the causative is formed by adding す／さす to the 未然形 of the verb
if the 未然形 ends in /a/, add す: 行く=>いか＋す＝行かす
if the 未然形 ends not in /a/, add さす: 見る=>み＋さす＝見さす
The resulting verb conjugates in 下二段, i.e.
And the causative of the verb す(equivalent of する) is often abbreviated: せさす=>さす
The modern causative forming せる／させる is simply the result of 一段化 of す／さす
This form is the most commonly used and is taught in textbooks as the standard causative form.
But there is another route to take which is 五段化
This form is rarely used and usually not covered by textbooks but it exists.
The first row of this form can be used to further form causative-passive, i.e. instead of 行かせられる you can say 行かされる (but ～さされる is not allowed).
Some godan-verbs apparently originate from causative, for example 飛ばす. But through usage they may have acquired other meanings than pure causative (try looking up いかす in 新明解).
I guess it goes without saying that not all su-ending verbs are causative in nature, for example 殺す(the vowel before /su/ isn't /a/) or 話す(no non-causative counterpart to be found).
If that's too much information, here's the shorter answer:
If the su-ending verb comes from the older causative form, then it's safe to say they mean the same thing.
Another entry させる (s.v.) is glossed as:使役。五段動詞は、行かす、飲ます、遊ばす、減す、載す、一段動詞は、見さす、食べさす、などとなる。
Examples given are self-deprecating forms derived from causatives with benefactive verbs:使役。謙譲語。一段動詞やカ変動詞の他、五段動詞の未然形につく。
Good. 言わす is a verb and 言わせる is 連語（使役） but they have the same meaning. を doesn't work with 言わす much but not impossible.causative of 言う (言わせる) and 言わす ?