Moved from Elongate-bodied Springtails.

You are right to exclude Cyphoderus.
Assuming it are Collembola, we have 4 candidate families: Hypogastruridae, Neanuridae, Isotomidae and Onychiuridae. I tend to think it are Onychiuridae which is a family of unpigmented Collembola, that lack eyes and furca, and that live deep into the soil.
If you still have access to live specimens, pls check whether or not these specimens produce yellowish drops of sticky fluid on their body when disturbed. Onychiuridae is the only family of Collembola that has such a defence mechanism.
But...
There are some things that are disturbing, confusing...
1. the first segment behind the head is relatively large. Way too large for Onychiuridae, Hypogastruridae and Neanuridae.
2. the terminal body segment is relatively 'heavy' ventrally.
3. the fact the specimens curl up when disturbed.
These 3 points remind me to Diplopoda. Diplopoda have 1st instars that have only 3 pair of legs. Even in adult Diplopoda the first 3 body segments still have only 1 pair of legs while all remaining body segments have 2 pairs of legs.
So my tentative conclusion is that it are first instar Diplopoda.
This is an interesting case that needs to be kept in both the Collembola and Diplopoda section of Bugguide. It is an excellent illustration how confusing determination of juvenile 'hexapods' can be.
Well done Stephen!