Originally this was an aside on the 725 Town Plan's page, but I found there was quite a bit to discover about this piece, so I moved everything here.

There are two recognized variants of the lamp. We'll cover them more in-depth later, but I want to lay the foundation first in case you're unfamiliar. This image is from BrickLink:

bricklink lamppost type a and b

It was first introduced in 1957 and was produced into the late 1960s. Two common variants are accepted to have existed, the thicker one labeled "Type A" and the thinner one labeled "Type B".

There also exists a commonly cited 1961 prototype of the lamppost on the 725 Town Plan's box. This is incorrect, and there are several ways we can know that. This is the specific one people are referring to:

lamp front 725 box

I've never really understood why it is thought to be a prototype. Why make the lamp point inward awkwardly? Why would a 1961 prototype of an existing piece have a different angle, especially if the both the earlier and later versions match 1:1?

Rather, I think this has a simpler explanation: its broken. I only have one version on hand, but the other is conveniently pictured on the box:

comparing the two lamp variants

Do you see how mine is thicker than the one used in the isometric? BrickLink's catalog entry(seen in the image above) identifies the thinner one as "Type B", implying that the thicker "Type A" variant is older.

I believe it to be the other way around. "Type B" came first, and we can know that for certain.

Take a look at the front of my 1959 700 set(it's a German town plan). BrickLink says the set is from 1957, but there are several versions of the set with different box tops. Mine is the 1959 version via the box top, though the contents imply an earlier date. My copy contains both parts with bottom tubes(new for 1958) as well as hollow bottom 2x8s and 2x10s. The plates also do not have the LEGO logo on the studs, which ceased in 1957. The box top for the 1957 version does not show the lamp post in the parts list, but mine does:

700 set box top full

Here's a closer look at both the rendering of the lamp and the physical piece featured on the box:

700 set box top rendering 1:87 accessories

700 set box top physical model

It's unmistakably the thinner version! So maybe it's possible the thicker one came out in 1957-58? No. As an example, any box art renderings of the VW Showroom, which released in 1957, show the thin version:

vw showroom box top

Even the sheet announcing the release of the streetlamps in 1957 shows a thin top streetlamp, meaning it is impossible for the thicker Type A variant to be the first:

From all that I could gather, we can conclude that at some point after 1961, the thinner one was replaced and redesigned to be thicker(as it simply wouldn't make sense the other way around, to make something strong much, much weaker). So to circle back, the reason for the redesign is quite clear—because it was so easy to break. So easy in fact that a broken one appeared on the 725's box top.

Now do you see what I mean? The alleged "prototype" is much more likely to be a broken lamp than an unknown variant.

The removal of the LEGO logo on the bottom is rather easy to explain. The original has the "dog bone" LEGO logo, which did not appear on packaging from at least 1961 onward. As that logo was antiquated, and it would also appear that the injection location on the physical piece had moved to the bottom on the thicker variant, the logo would have been a candidate for an update—or outright removal in this case. Other 1:87 elements(Esso signpost and gas pumps come to mind) do keep the dog bone logo on their bottom into the late 1960s (late 60s redesigned prototypes have been shown, neither of which keep the old dogbone logo).

I don't know that BrickLink intended to imply that the "Type A" variant came first, but now we know and can also demonstrate that the "Type B" variant came first. Then it made it at least into the 1960s before being redesigned and replaced with the thicker "Type A" variant.