Looking for a PCB flag/indicator component

There are non-volatile electromagnetic indicators that can be used.

They are a little oddball though. They were originally developed by the Canadian unit of Ferranti-Packard, but the patent has long since run out.

They work like a bistable relay (electromagnetic) and retain their state with power off. In the old days they were used in quantity for airline status displays in airports.

Personally I would consider fitting the trays with batteries (or applying power later) and using LEDs. You could store the information in a non-volatile memory such as a small microcontroller with on-board EEPROM or FRAM if the power has to be interrupted.

The flip dots mentioned in other answers would probably be more suitable if you can still find them, but another possibility is an E-Ink display: they're available in forms with individual dots, rather than the usual dot-matrix or 7-segment form. That's a completely non-volatile display, that can be directly driven by 7 pins of a 5V microcontroller, no additional circuitry required. (I'm not sure if 3.3V would be enough to get usable contrast. These displays want 15V for full performance, but the main issue with using only 5V is that it takes about a second to change state.)

I was running on a similar train of thought as Spehro.

Figure 1. Magnetic latching indicators - flip-disk and flip-dot displays. Source: eldisrl.com.

These are similar to the elements that make up seven-segment displays on some petrol pumps.

Figure 2. These paraticular units have a two-wire coil and flip state is determined by polarity reversal. Source: Datasheet.

Figure 3. A manually reset and electrically unlatched indicator was used on the old manual switchboards. Source: Telephone switchboard, Wikipedia.

They were also used for call-bells but I can't find an image of anything modern. Would you 3-D print your own with a little solenoid release mechanism?