Watchman
I tend to agree with your approach to scripture: but in this case I think you're flogging a dead horse.
The AV perfectly correctly puts the word 'gifts' in italics, meaning that there is no corresponding greek word for 'gifts' just there. But that may merely be an ellipsis, rather than necessarily attaching 'spiritual' to 'brethren'.
'Spiritual' could be legitimately attached to 'brethren', but I think the most sensible way to proceed, is to examine what the different translators produce here.
The AV may be wrong - it usually isn't, but it could be. So rather than enter into too much rancour and production of ill-feeling, here is a listing of the various translations I have in the Online Bible. I hope it helps.
1 ¶ Now concerning spiritual gifts, brothers, I would not have you ignorant. AKJV
1 ¶ Now concerning spiritual gifts, brethren, I would not have you ignorant. ASV
1 ¶ But about the things of the spirit, my brothers, it is not right for you to be without teaching. BBE
1 ¶ but, brothers, I do not want you to go on being ignorant about the things of the spirit. CJB
1 ¶ Now concerning spiritual gifts, brothers, I do not want you to be ignorant. EMTV
1 ¶ Now concerning spiritual gifts, brothers, I do not want you to be uninformed. ESV
1 ¶ It is important, brethren, that you should have clear knowledge on the subject of spiritual gifts. WEYMOUTH
1 Now, concerning the spiritual gifts, brethren, I do not desire you to be ignorant. WUEST
There must be others, but I don't have them.
They seem unanimous on the point, and I think it's the context that drives their rendition.
If you look down the passage, there shortly follows a huge series of things which look suspiciously like 'gifts'. In fact there are a few 'givens' in them:
4 Now there are diversities of gifts, but the same Spirit.
5 And there are differences of administrations, but the same Lord.
6 And there are diversities of operations, but it is the same God which worketh all in all.
7 But the manifestation of the Spirit is given to every man to profit withal.
8 For to one is given by the Spirit the word of wisdom; to another [is given - an ellipsis] the word of knowledge by the same Spirit;
9 To another [is given - an ellipsis] faith by the same Spirit; to another the gifts of healing by the same Spirit;
10 To another [is given - an ellipsis] the working of miracles; to another [is given - an ellipsis] prophecy; to another [is given - an ellipsis] discerning of spirits; to another [is given - an ellipsis]divers kinds of tongues; to another the interpretation of tongues:
11 But all these worketh that one and the selfsame Spirit, dividing [= giving] to every man severally as he will.
So with all that before us, we can easily comprehend why the translators almost unanimously insert gifts into the passage.