Did they only manufacture one type of these? I have been digging around a little and found a few things.
One place was advertising
A.R.M.S.® ITEM #39 A2Plus Mount
A different place, www.ustacticalsupply.com had theses 3
Item #39 Std:
Item #39 Ext:
Item #39 S-Ex
I am using a Hakko right now and they are pretty long. I bought an el cheapo imitation #39 to try out and the bottom rail is not long enough for the sight to sit fully in place w/o hanging off the front, so if the 39 was offered in an extended version, I pretty much know it would be long enough.
The only #39's I have ever seen are only made in one length, and it goes out to the end of the carbine hand guards. Jack