S. R. Faruqi:

The similitude between a grave-tablet and a mirror is that both are usually rectangular, and both are placed within a frame. In [the Persian dictionary] bahār-e ʿajam 'tablet' is given as a simile for a mirror. Similarly between mirror and tablet there's also an affinity of meaning. A mirror in which the whole figure can be seen is called āʾīnah-e badan-numā . That is, what we call a full-length mirror [qadd-e ādam āʾīnah], has the technical name of āʾīnah-e badan-numā .

Against this background, the theme arranged by Mir's imagination makes one shiver with fear. Mankind's end is death. The grave-tablet is a symbol of that death, or a sign of mankind's final dwelling-place. Thus in the grave-tablet mankind's end can be seen. It's obvious that the eye doesn't see anything, but people who have the eye of insight know the grave-tablet to be a full-length mirror. For this reason they don't understand self-regard and self-display, because in the mirror they see their own end.

For them, a full-length mirror is like those magical mirrors [in story tradition] in which instead of the body, only a heap of bones can be seen. They know that the person who is reflected in the mirror is only suppositional and artificial, and flesh and blood, color and vitality are only superficial affairs. They also know that this form will not remain established forever. Whatever is now seen will all melt away, and only a pile of dust will be left. And then, how long will this pile of dust endure? When their eyes would see all this, then how would there be any self-display in the mirror?

In Oscar Wilde's novel 'The Picture of Dorian Gray', Dorian Gray himself doesn't get older, but his picture becomes old. In Mir's verse, to the people of insight a youthful image too appears old.

The affinity between 'mirror' and tablet' has already been suggested above. In addition to this, the following instances of wordplay are also enjoyable: badan-numā , ḳhvud-numāʾī (the mirror is 'body-displaying', but it causes no 'self-display'). Then, turbat , ḳhāk ; and badan , ḳhāk ; and naz̤ar , ḳhvud-numāʾī ; and āʾinah , ḳhāk (a mirror is cleaned by being scrubbed with dust).

In fact lauḥ is feminine, but through the affinity with 'mirror' he has written turbat kā lauḥ . In Mir's time, this was not incorrect. On this, see:

{17,8}.

After Mir, up until the time of Zauq they wrote it this way:

daryā-e ġham se mere gużarne ke vāst̤e
teġh-e ḳhamīdah yār kī lohe kā pul huʾa

[with regard to my passing over the ocean of grief
the friend's curved sword became an iron bridge]

FWP:

SETS
MOTIFS == MIRROR
NAMES
TERMS == WORDPLAY

In case you haven't seen any grave-tablets, here's a group beside Ghalib's tomb that gives the general idea.

A bleak verse, but enlivened by the kind of wordplay that is also most unforgettably 'meaning-play' as well.