The GSTIN can checksum and still be the wrong PAN.

Acme Steel Fittings. You have posted to their ledger for three years. The GSTIN on the master is 27AAPFU1234A1ZF. The PAN is AAPFU1234A. Then a new supplier appears: Acme Steel Fittings, same rubber stamp, same product, different account. Their GSTIN is 27AAPFU1243A1ZE. The PAN is AAPFU1243A. One digit different. Checksum still passes. Both look like "Acme." Both look like "the same GSTIN." They are not the same company.

A well-formed GSTIN is 15 characters. State, PAN, entity, Z, checksum. The anatomy post walks the checksum. This page is the slice people skip: the PAN in the middle can be a different company than the ledger you have been posting to for three years, while the letterhead still says "Acme Steels."

Why this happens on paper

Sister concerns, same brand, different PAN. A clerk copies last month's GSTIN onto a new entity's invoice. OCR reads a 4 as an A inside the PAN and the checksum still works for the new string. Group companies in two states: same PAN, different GSTIN (entity/state). That last case is legitimate. Different PAN is the dangerous one.

Portal legal name vs trade name on the photocopy: they often disagree even when the GSTIN is right. Do not use "name fuzzy match 90%" as PAN proof. Compare the ten characters.

OCR substitutions inside PAN (8/B, 0/O, 1/I) often still produce a checksum-valid string that belongs to someone else, or to nobody. Portal search on the extracted GSTIN tells you the legal name. If that name is not your vendor, you have the wrong number even if checksum passed. Status cancelled is a different post.

What we run today

Checksum on seller and buyer. Regime from state prefixes and POS. Duplicates on invoice number plus seller GSTIN. We do not compare scan PAN to Tally master PAN in validate_invoice. If a salesperson says we do, they are wrong. Master compare is a review process, or a future check. Do not write it into an MSA as if it shipped.

Same-PAN, two GSTINs (MH and KA plants) should both checksum. Uniqueness is still invoice number plus that GSTIN, not plus PAN. Two plants can issue INV-1. That is not a duplicate under our key. Link duplicates.

What the clerk should do

On review, show PAN slice of the scan next to PAN slice of the mapped ledger. Equal: continue. Unequal: stop. Confirm on gst.gov.in which legal name belongs to the scan GSTIN. Then either map a new ledger or send the paper back. Do not post to the old Acme ledger because the stamp looks familiar.

Seeding mappings in your workspace is how you stop nightly surprises. Link import. Mapping is name-to-ledger. This page is GSTIN-to-entity. You need both. Name match without PAN match is how sister-company ITC lands in the wrong place.

Entity digit is not PAN. The character after PAN is the entity number for another registration under the same PAN. 27AAPFU1234A1ZF and 27AAPFU1234A2Zx are related, not strangers. Different PAN in the middle is a stranger. Show the ten-character slice in isolation so the eye does not slide off the first 27.

Master hygiene

Tally ledgers collect extra GSTINs over years: old registration, new state, typo that someone "fixed" by adding a second master. Import mapping to the wrong one is silent. Seed one GSTIN per ledger in your workspace and refuse a second without an accounts-head note. We will not clean their masters for them.

ITC follows the GSTIN that filed. If you posted against the wrong GSTIN, 2B will not match that voucher. Section 16 does not care that the letterhead was similar. Cancelled vs wrong-PAN are different portal facts. Link cancelled GSTIN.