pattern of alternating betweeniambs ( ^/ ) and trochees ( /^ ), though one line is in dactylic ( /^^ ), with metrical feet that favor tetrameter and pentameter.

Several lines have a dropped final unstressed syllable in the trochaic rhythm, final dropped weak stresses are called catalexis, and a couple of lines have an unorthodox dropped strong stress at the end of an iambic rhythm, while there are several instances in which a pause/comma is used to fill a weak stress (lea' ther,  / pa' ten ted)--which is a usage in keeping with one of the original, though often overlooked, features of poetry in the English language--or two syllables are slurred to form one (flake' from_the / wall'). Finally some lines change in mid-line from iamb to trochee or in reverse order.

Iambic line: As es' / ti ma' / ted,  / you died'. / Things marched',

Trochaic line: or' passed / o' ver / at' the / pro'per / time'.

Dactylic line: Just' so much / Zy' klon and / lea' ther,  / pa' ten ted

Dropped final weak stress: been', un / touch'a / ble'

Dropped strong stress: an^ el' / egy^ for' / my^ self' / it^

Change in mid-line: of harm' / less fires' / drifts' to / my eyes'.

The meaning of the poem is as follows with a progressive commentary:
Undesirable you may have been, untouchableyou were not.
This indicates a person who for a reason later revealed was not desired by society but was nonetheless dearly beloved by the poetic speaker/persona.

Not forgottenor passed over at the proper time.
It is someone not mentally or emotionally overlooked at their time of death.

As estimated, you died. Things marched,sufficient, to that end.
The person's death was not unexpected. It was anticipated. "Marched on" is an indicator of what is being spoken of.

Just so much Zyklon and leather, patentedterror, so many routine cries.
This reveals who is being spoken of. Zyklon was a gas used in the death chambers of Nazi concentration camps. This is a painfully sarcastic, dismissive remark explaining why the death was expected: The individual being eulogized was just one more undesirable Jew to be gassed by leather-wearing soldiers who made up the "patented" terror of death. "Patented" is a triple play on words meaning (1) patent leather, (2) patented as in an invention being given a patent, (3) patent as in obvious as in the patent truth. "Routine cries" systematizes and dehumanizes the cries of suffering coming from the Zyklon gas-houses.

(I have madean elegy for myself itis true)
The writer too expects the same sort of end and has prepared his parting death song in advance.

September fattens on vines. Rosesflake from the wall. The smokeof harmless fires drifts to my eyes.
These are signs of life and renewal around him, signs that in some measure wouldn't be good enough for anyone else.

This is plenty. This is more than enough.
These tokens of life are enough for a reminder of joy and a sense of life.

"September Song," by Geoffery Hill, seems to be a poem written in honor of a victim of the Nazi extermination of Jews during World War II.  The clearest hint to this is the mention of Zyklon, which is the poison gas that the Nazis used to kill millions of people in concentration camps.

The victim is a child who was born in 1932 and killed in 1942, just a few months past his or her 10th birthday.

The poet describes the victim as "undesirable," but not "untouchable"; i.e. the Nazis did not desire this person, but they knew where to find him and how to "touch" him or kill him.

The Nazis were known to be very orderly and systematic in the way that they exterminated people.  This is hinted to in the lines:

As estimated, you died.  Things marched,

sufficient, to that end.

In other words, you died just as the Nazi "scientists" estimated that you would upon being exposed to a particular dose of zyklon.  "Thing marched...to that end," meaning that the process of capturing and then killing innocent people "marched" to its desired conclusion.

The poem is an elegy, meaning that it is a tribute to a deceased person.  The victim is not named, just as his murderers took no interest in his personal attributes.