The over bar you are talking about is ONLY used in Sahidic dialect, and it is used for the reason you mentioned
in bohairic the jenkem is like this ` and is also used for the same reason
but in Sahidic it is important since it frequently replaces the ⲉ
for example ⲙ͞ⲛ for ⲙⲉⲛ (ⲛⲉⲙ in Bohairc), in Bohairic we never do that
Well, I thought everybody knows that jenkem (in bohairic) is a new addition
but when you said that you didn't know, I searched for manuscript (online) to double check
As for Sahidic Dialect, I could confirm that this "bar above" existed even before the 4th century
In Bohairic, I found jenkem in some manuscripts during and after the 10th century
I couldn't find any readable Bohairic manuscripts before the 10th century except this one http://www.biblical-data.org/coptic/926a.jpg
which is dated to be in the 4th century
It has no jenkems in it, However I think one manuscript is not enough to prove anything
The manuscripts I found are located here http://www.biblical-data.org/coptic/coptic_MSS.html
Please let me know if you could get more info about this topic
I have no problem with adding the word with jenkem as a variation, it doesn't hurt
But what about the examples?
crum doesn't use/mention this jenkem at all in the examples what do you think?