I got asked this question by Bartfeels24 on Reddit (see below). This is my reply, if you are still wondering about the why?:
Equal (3): Everything in this space must be equal to 3. The answer is 3-1, placed vertically; 4-3, placed vertically; 0-3, placed vertically; 3-3, placed vertically.
，推荐阅读91吃瓜获取更多信息
Continue reading...，更多细节参见谷歌
The secondary mirror is adhered to the top yellow bracket (groove supported for ease of positioning) using standard strong adhesive. To ensure wearing gloves while handling the mirrors.