The first synchros for 3rd and 4th are easy. The split case gearbox from 53 to 60 used synchro rings 113 309 295. AFAIK, this is the only part number used for the entire run of that gearbox.
Here's a pic of a couple of these synchros. Note how thin it appears compared to a later synchro.
When the tunnel type full synchro gearbox appeared in Aug 60 for the 61 model year, it used synchros with part number 113 311 295. This synchro has a large chamfer on the OD end opposite of the dog teeth. It needs this chamfer so it can fit in the recess of the 3-4 clutch gear. In Nov 61 (axle number 4 371 905) the inside of the clutch gear was opened up so that this chamfer was not needed. In Mar 62, a new synchro without this chamfer appeared, 113 311 295A. This change was done so that the clutch gear can better center the synchro ring. The new 295A synchros cannot be installed with the earlier 3-4 clutch gear.
Here's a back-to-back comparison pic of a 295 on the left and a 295A on the right.
We are all familiar with the currently available synchros, 113 311 295D. These have the 4 large notches grouped together to provide an imbalance that according to VW reduced howling when the gearbox was cold. They also have 3 small notches in their OD to indicate the angle of the tops of the dog teeth, 120°.
Here is what Long Enterprises has to say about the change from 110° to 120°:
Notice how they skip from a 295A to a 295D.Long Enterprises wrote: Tech Tip #2 Synchros: 110° vs 120° Face Angles
Question: What is the difference between synchros with the 3 notches on the side (113 311 295D) and the ones without (113 311 295A)?
Answer: The 113 311 295D synchros with three notches on the sides have 120° face angles and the 113 311 295A (no notches) have 110° face angles.
The 113 311 295D have synchronizer teeth with 120° degree face angles. When these synchros were first introduced the face angles on the engagement teeth of the sliders were also changed to 120°. The additional 10° more gradual angle provided improved shifting. The first 3-4 slider with 120° face angles on the engagement teeth was introduced at the same time (late 1971) and can be identified by two annular grooves, one on either side of the shift fork slot. This 3-4 slider requires a 113 311 295D synchro (120°) on both 3rd and 4th gears. The 002 311 315 3-4 slider (72-75, 5mm shift fork) uses the 113 311 295D synchro on 3rd and 4th also.
In the orange Bentley manual on page 35 of the transmission section, it states:
Fig 9-37 has an arrow pointing to the 4 large notches grouped together that we've all seen.Mr Bentley wrote:
Beginning in 1972, grooves were machined into one part of the 3rd and 4th gear synchronizer rings. This gives the ring an imbalance that, in conjunction with centrifugal force, keeps the ring in continual contact with the gear wheel. This helps prevent 4th gear howling, which sometimes resulted from a cold transmission. See fig 9-37.
Then they go on to say:
Mr Bentley wrote:
Midway through the 1972 model year, a second kind of redesigned synchronizer ring was introduced in production. This ring resembles the one that was previously shown in fig 9-37, but is for use with 4th gear only. Subsequently, a redesigned 3rd gear synchronizer ring made its appearance. The new 3rd gear ring was introduced gradually and, therefore, may not be found except in transmissions manufactured later in the 1972 model year. However, both of these redesigned synchronizer rings can be used as replacement parts in earlier transmissions.
The latest-type synchronizer rings should always be used for 4th gear repairs. The earlier-type 3rd gear rings, however, are adequate as replacement parts if the latest-type rings are not available.
I've read this Bentley quote over several times and I understand it to say there are 3 different synchros that all appeared within a few months of each other. So far we've got two changes: the 4 large notches and the 120° dog teeth. What is the third change? What are all the part numbers?
In my inventory of synchro rings, I have:
113 311 295
113 311 295A
113 311 295C
113 311 295D
Here's a 295C and a 295D. Both have the imbalance notches, and the 120° indicator notches. Given that, what is the difference between them?
Is there a 295B? What's it's design change?