Trust by verify. Below are my calculated drop rate distribution per item for 108 Engineer boxes, 100 purchased and 8 from missions or drops:
| Item Qty | Item | Qty | Calculated Drop % | WG Drop % Bin |
|---|---|---|---|---|
| 100000 | Credits | 12 | 11.1% | 100% |
| 250000 | Credits | 3 | 2.8% | ” |
| 250 | Gold | 5 | 4.6% | ” |
| 500 | Gold | 5 | 2.8% | ” |
| 1 | Personal Training Manual | 1 | 0.9% | ” |
| 1 | Crew Book | 6 | 5.6% | ” |
| 3 | Additional Experience Booster, 50% 1hr | 13 | 12.0% | ” |
| 5 | Additional Experience Booster, 50% 1hr | 5 | 4.6% | ” |
| 3 | Credit Booster, 50% 1hr | 7 | 6.5% | ” |
| 5 | Credit Booster, 50% 1hr | 3 | 2.8% | ” |
| 5 | Demount Kit | 6 | 5.6% | ” |
| 5 | 2D Style | 5 | 4.6% | ” |
| 1 | Day Premium | 16 | 4.6% | ” |
| 3 | Day Premium | 7 | 6.5% | ” |
| 5 | Day Premium | 3 | 2.8% | ” |
| 1 | Event Decal | 12 | 11.1% | ” |
| 1 | Engineer Starter | 2 | 1.9% | 5% |
| 1 | 3D Style | 3 | 2.8% | ” |
| 1 | Premium Vehicle | 3 | 2.8% | 2.4% |
Overall, WG was accurate with their overall distribution percentages 
Clearly there are certain items more likely than others in the guaranteed set, expect 100,000 credits, 3 Additional Experience Boosters, and a single day of premium time as most likely outcomes (~12%). You will know you are lucky if your receive a Personal Training Manual Personal Training Manual as it is least likely to drop item (1%).
I am open to more data if anyone has a complete data set.
Source: https://www.reddit.com/r/WorldofTanks/comments/pwsuix/drop_rate_of_engineer_boxs/
