[gundos]

Here's what's posted on that forum:
The Arabic part says that this is narrated by Shaikh Zubaidy.

These are my assumptions/thoughts:
1. It may be a door for maintenance
2. It may be a marking that it is above the blessed Prophet, so that the people who work on the dome do not step over it (for painting, washing, etc)
3. Is Sheikh Zubaidy, who narrates the story, a well-known Alim? Who is he? Sunni or Shia, etc?
4. Maybe if we know the history of construction of the dome, we can judge the authenticity of the lightning story?
5. It may be a lightning conductor (I suspect it is not)

والله اعلم

Slight history:

The history of the green dome

The dome over the grave of the Prophet (peace and blessings of Allaah be upon him) dates back to the seventh century AH. It was built during the reign of Sultan Qalawoon, and at first it was the colour of wood, then it became white, then blue, then green, and it has remained green until the present.

Professor ‘Ali Haafiz (may Allaah preserve him) said:

There was no dome over the sacred chamber. There was in the roof of the mosque above the chamber a waist-high enclosure of brick to distinguish the location of the chamber from the rest of the mosque’s roof.

Sultaan Qalawoon al-Saalihi was the first one to build a dome over the chamber. He did that in 678 AH. It was square at the bottom and octagonal at the top, made of wood, and built on top of the pillars that surrounded the chamber. Planks of wood were nailed to it, over which plates of lead were placed, and the brick enclosure was replaced with one made of wood.

The dome was refurbished at the time of al-Naasir Hasan ibn Muhammad Qalawoon, then the leaden plates slipped, but they were fixed and refurbished at the time of al-Ashraf Sha’baan ibn Husayn ibn Muhammad in 765 AH. It fell into disrepair and was renovated at the time of Sultaan Qayit Bey in 881 AH.

The chamber and dome were burned in the fire that swept through the Prophet’s mosque in 886 AH. During the reign of Sultaan Qayit Bey the dome was rebuilt, in 887 AH, and strong pillars to support it were built in the floor of the mosque, and they were built of bricks to the correct height. After the dome had been built in the manner described above, cracks appeared in its upper part. When it proved impossible to refurbish it, the Sultaan Qayit Bey ordered that the upper part be demolished and rebuilt strongly using white plaster. So it was built solidly in 892 AH.

In 1253 AH, an order was issued by the Ottoman Sultan ‘Abd al-Hameed to paint the dome green. He was the first one to colour it green, and the colour has been renewed whenever necessary until the present.

It became known as the green dome after it was painted green. It was previously known as the white dome or the fragrant dome or the blue dome. End quote.

Fusool min Tareekh al-Madeenah al-Munawwarah by ‘Ali Haafiz (p. 127, 128).