Part number is the item
This trade runs on part numbers, and the counter enquiry is answered in seconds or it is answered by a catalogue. Keeping the part number as the item code — searchable and scannable — is what makes that possible. Make and model belong on the item too, because that is how a customer asks, and it is what prevents the wrong part being sold and returned.
Genuine, OEM and aftermarket
The same part exists at three quality levels and three price points, and a shop entering them as one item can track neither stock nor margin. Each as its own item, clearly named, also protects the conversation at the counter: the customer chose the cheaper one and the bill says so, which matters when it fails in eight months and they return.
Mechanic accounts
Your regular buyer is a mechanic drawing parts for jobs he bills at the end of the week or month — small amounts, high frequency, and a running account nobody totals. The ledger building itself from the billing means it is recorded without effort, and a weekly or monthly statement settles it before it becomes a number both sides argue over.
Seasonal demand
Tractor work concentrates around the crop cycle, and parts demand follows it — sowing and harvest weeks look nothing like the months between. Movement reports over the same period last year tell you what to hold, and minimum levels on the fast-moving parts stop a mechanic being sent elsewhere because you were out of a filter that costs a few hundred rupees.