Where are the "World's First Space Engines" now?

Question

From the Wikipedia article Pioneer P-30:

At an altitude of about 370 km (230 mi), the first stage separated from the second stage. When the second stage was ignited, telemetry showed abnormal burning, and the stage failed due to a malfunction in the oxidizer system. The vehicle was unable to achieve Earth orbit, re-entered, and was believed to have come down somewhere in the Indian Ocean. Signals were returned by the payload for 1,020 seconds after launch. The mission was designed to reach the Moon approximately 62 hours after launch. Although the mission was a failure, ground controllers fired Able VA's onboard liquid propellant hydrazine rocket engine — the first time that an onboard motor was fired on a space vehicle.

The 2010 post Topic: Able IV: World's first space engine (1959) at Collect Space dot com give a sighting of an un-flown engine in 1970.

Since some were launched, and one even ignited in space successfully, there must be, or have been at least one on the bottom of the ocean (unless there were heroic recovery efforts).

Question: Where are they now? Are there any currently on public display? Were any of these historical items ever recovered?

"Bonus:" Why is it notable that this started in space, and why is it different than second stages (including the one in this mission) starting in space? What is the "firstness" here?

In 1960, STL built two more hydrazine monopropellant space engines of the same design and designated Able V-A and Able V-B. These engines were launched September 25, 1960 (Pioneer P-30) and December 15, 1960 (Pioneer P-31). Unfortunately, both launches failed shortly after lift off. However, on the 17 minute Pioneer P-30 mission, the Able V-A engine got to see action. Despite a subpar second stage burn which prevented the Pioneer from reaching escape velocity, ground controllers were still able to fire the Able V-A engine. Therefore, STL’s Able IV/V space engine design “was the first to successfully ignite and operate in space.”

Descendants of the Able IV engine are still in use today. Thanks in part to the pioneering efforts of STL, hydrazine engines can be found maneuvering the International Space Station, the Space Shuttle, communication satellites, and more.

TRW presented the remaining Able IV space engine to a retiring VP in 1970.

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There are are archived goodies for reference material - see here also.

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below: Abel, the world's first space engine.

Answer 1

The only surviving model of the Able IV spacecraft is privately owned and not in public display. As mentioned in an article of the Explorer magazine published by the American Astronautical Society (AAS) (Issue 10, March 2010, page 7):

Robert Enichen of Yardley, Pa., is the proud owner and humble caretaker of a piece of space history — one of the first series of liquid-fueled rockets to fire in space. Originally, TRW presented this 1959 vintage Able IV space engine to retiring TRW Vice President Ed Robinson, an avid engine collector. Enichen acquired the space engine after Robinson decided to wind down his collection. Enichen graciously provided pictures of his engine aswell as an article on its history.

As you mentioned in your question Able V-A and V-B were on board launches that failed, so most probably they were destroyed on the launch pad (in the case of V-B) or crashed in the ocean (in the case of V-A) and never found.

You can see a picture of the Able IV spacecraft bellow:

This and other photos can be found at Robert Enichen's website.

As for the question of the difference between Able's engine and a common second stage engine, the answer can be found in his website:

The monopropellant hydrazine (N2H4) fueled Able IV engine was capable of multiple restarts in space allowing midcourse velocity correction and retro propulsion to adjust the spacecraft’s trajectory en route to the planned lunar orbit. Previously, rocket engines burned continuously until their fuel was exhausted.