The Q4018420 is the steel Master Control Date, with a 40 mm case and a date. The Master line goes back to 1992, when Jaeger-LeCoultre presented its watches with a thousand-hour test after assembly.
The Master Control Date is its simplest form: hours, minutes, seconds and a date, in a round case with a restrained dial.
Inside is an automatic in-house calibre from Jaeger-LeCoultre, calibre 899AC, visible through the display back.
Precisely because the watch is so restrained, the details count on a pre-owned example. We look at the alternation of polished and brushed surfaces on the case, which blurs after inexpert polishing, at the dial and hands, and at the condition of the strap and the buckle or folding clasp.
We test the automatic winding, the setting of time and date and the date change around midnight, and we compare the reference and serial number with the papers. If you would like a Master with a moon phase, have a look at the Master Ultra Thin Moon Q1368420.
How we assess a watch you are selling is explained in our guide to having a watch valued.