Indeed, your test results are more conclusive and demonstrate my point! If there were in fact a hidden effect, the damage of the Secret Dagger +4 would be higher than the damage of the Dagger +5. However, as shown by the data above, the weapons have the exact same attack rating and therefor result in the exact same backstab / reposte damage.
The only reason why a Secret dagger has higher backstab/reposte damage when comparing columns side-by-side is because it has a higher attack rating overall. Higher attack rating = Higher backstabl/reposte damage.
Theres nothing secret about that.
Therefor it is false to advertise this weapon as having a bonus to the backstab damage based on this information that we both have gathered.