The 423001 is the 50-watt high-pressure sodium lamp that goes in a Tri Lite HPS dock light head. The workbook describes it as "HPS Head Only", which is the important part: you are buying a bulb, not a light.
Price is $99.99. The head, the ballast and the ignitor are not in the box, so if what failed is the head rather than the lamp, this will not fix it.
One confusion to head off straight away. There is a Tri Lite 423010 PAR38 LED bulb at $149.99 that sits one digit away in the catalogue. It is a completely different lamp for a completely different head. Check which one your fixture takes before ordering.

What does the 423001 bulb replace?

The 50-watt high-pressure sodium lamp inside a Tri Lite HPS lamp head. HPS lighting puts out that characteristic warm amber light and needs a ballast and ignitor to run, both of which live in the head rather than the bulb. When an HPS dock light starts cycling on and off, or will not strike at all, the lamp is the usual culprit and the cheapest thing to try first.

Which heads does the 423001 fit?

Tri Lite HPS lamp heads, including the HPSHEAD-P where specified. The manufacturer documents the HPSHEAD-P as a 50-watt high-pressure sodium assembly, and distributor parts lists identify 423001 as the replacement lamp for HPS heads. Before ordering, read the part number off the head itself and confirm it is actually an HPS fixture. A dock light head that takes a screw-in PAR38 lamp is a different product entirely, whatever it looks like from the ground.

Is the 423001 an LED bulb?

No, and this is the mistake worth avoiding. It is a high-pressure sodium lamp. The Tri Lite 423010 is the PAR38 LED bulb, listed separately at $149.99, and the two are not interchangeable in either direction. An HPS head needs a ballast and ignitor that an LED lamp has no use for, and a PAR38 screw-in head has nowhere to put an HPS lamp. The part numbers differ by one digit, which is exactly why orders go wrong.

Does the $99.99 include the lamp head?

No. The source description says "HPS Head Only", meaning this bulb suits an HPS head and nothing more is supplied. You get the lamp. If the head is cracked, corroded or has a failed ballast, you are looking at a different purchase, and replacing a bulb into a head with a dying ballast will have you back on the same ladder in a fortnight.

How do you check you are ordering the right part?

Three steps, all at the fixture. Read the current light head's part number. Confirm the wattage it is rated for. Confirm it runs an HPS ballast rather than another lamp technology. If the head has been replaced at some point and the paperwork says one thing while the label says another, believe the label. Ten minutes with a torch beats a wrong $99.99 part and a dock bay still in the dark.

How long does an HPS lamp last?

Longer than people expect, though we will not put a number on this particular part without a datasheet to back it. The 24,000-hour figure that circulates describes the bulb supplied with the HPSHEAD-P, which is not the same as a verified standalone specification for 423001. If a rated life matters to your maintenance planning, ask for the bulb datasheet rather than working from the head's published figure. HPS lamps also dim gradually rather than failing outright, so a lamp still lighting is not necessarily a lamp still doing its job.

Who buys this part?

Dock and facility maintenance, keeping existing HPS dock lighting running. HPS heads are still widespread because they were installed in quantity and they last, and the economics of replacing a working head to gain an LED are not always obvious. A $99.99 lamp that brings a bay back tonight is often the right call, even on a site planning an LED programme for next year.