Precharged rechargeable batteries will discharge in storage over time, self discharging design faster than non self discharging design.
Time frame from manufacturing charge until you received them will likely remain unknown.
Rechargeable batteries usually have a lower voltage than their alkaline counterparts at full charge. NiCad's were much lower and cameras have a switch for Nicad or LR6.
The most overlooked feature of rechargeable batteries is their current rating usually stated in millamp hours. This is the amount of current that the battery can supply in one hour.
https://en.wikipedia.org/wiki/AA_battery shows Alkaline to have 1800 to 2600 mAh while Survival Frog USB rechargeable
Dead Link Removed have 1450 mAh rating with no statement of their full charge voltage.
http://www.cameramanuals.org/flashes_meters/minolta_auto_320_320x.pdf shows the flash should operate on NiCad batteries so check that your rechargeable batteries are fully charged before use.