Open Back Sperzel information:
Back to Custom Sperzel tuners
These new keys feature "friction locking" string  post's.
They use no mechanical locking devices whatsoever
(no tools, mechanical, or "moving parts", are used in the locking post system).

They lock by using -3- uniquely milled and drilled "friction points" within the string posts:

Friction Point 1: Is caused by the string slightly bending in the hole that passes down  through the post.
Friction Point 2: Is caused when the string bends slightly out of the downward post  hole and then sits in the slot on the top of key.
Friction Point 3: Is caused when the string bends one more time out of the slot , and then wraps 1/2 to 3/4 around the post before traveling to the instruments nut slot.  

To string them up:

First,  turn all the buttons (knobs) until all the tuner post holes (the small angled holes on the posts  that the strings pass through) are facing AWAY from the nut, and the post slots are in-line with the travel of the strings.
Then, just run the string all the way down through the angled hole in the edge of the post   (the string is pulled all the way out the back of the key), while also making sure the string sits in the slot on the top of the string post. .
Then while pulling tight the excess string slack that hangs out of the back of the key (with one hand), use your other  hand to tune the string  to pitch , (trim of the excess string at the back of the tuner gear)...
Stringing up these keys in this way, causes the 3 point friction locking system to take affect.
This is WAY easier to do then it is to try to read about here (it's even easier then stringing up the thumb locking Sperzels)...
It's really that simple!

Open-Back Sperzel compared to a standard thumb-locking Sperzel:
Here is a top view photo of the two side by side to give you an idea of where the mounting pins are located.
As you can see the Open-back key has it's set pin much closer to the post, when compared to the standard "thumb-locking" Sperzel.

However, both string posts will fit in the same sized head stock posts holes...
The open back set pins are just -slightly- larger then on the "thumb-locking" keys,
but I would still use a 7/64" drill bit (same bit I use on the thumb locking Sperzels) to drill out the mounting pin holes..

Both keys have the same 12:1 gear ratio.