AdDuplex returns with another monthly peek at Windows Phone handset usage. This month, the Nokia Lumia 920 continues its reign as the top Windows Phone handset. But the big news is the arrival of the budget Lumia 520, which has already jumped into 8th place in its first month on the chart, and the first sightings of test Windows Phone 8.1 builds.

As you may remember, AdDuplex bills itself as the largest cross-promotion network for Windows Phone and Windows 8 apps, empowering developers and publishers to promote their apps for free by helping each other. And each month it provides a tantalizing glimpse at which Windows Phone (and Windows) devices people are actually using.

Some trends from this month’s report, which will be live sometime this week:

Lumia 920 strengthens its lead. Last month, the Lumia 920 jumped into first place as the most-frequently-used Windows Phone handset. This month, the 920 strengthens its lead by one full percentage point over the declining Nokia Lumia 710.

Top 5 are all Nokia. Once again, the top five Windows Phone handsets are all made by Nokia: Lumia 920 (13.1 percent), Lumia 710 (12.1 percent), Lumia 800 (11.2 percent), Lumia 610 (8.4 percent) and Lumia 620 (8.1 percent). Actually, the top six are all Nokia devices: The Lumia 820 took 7.3 percent of usage, well ahead of the 7th place finisher, HTC Windows Phone 8X.

Budget phones surge. Last month, AdDuplex noted that it expected to see Nokia’s budget phones surge as new models like the Lumia 520 and 720 hit the market. That’s happening: The Lumia 620 jumped from 7 percent of usage last month to 8.1 percent this month. And as noted previously, the new Lumia 520 entered the charts in 8th place with 4.4 percent of the usage. (The Lumia 720 enters in 14th place with 1.7 percent usage.)

Nokia dominance has plateaued. It appears that 80 percent of the market is the natural plateau for Windows Phone’s dominant player, Nokia. This firm has controlled about 80 percent of the market for several months now.

Nokia not as strong in the US. As noted in Nokia’s recent quarterly earnings announcement, the firm continues to struggle a bit in the US. So while Nokia devices control 80 percent of Windows Phone usage overall, they only account for 66 percent in the US, with HTC grabbing 26 percent (well above its global share of 13.7 percent), thanks largely to the success of the Windows Phone 8X. Samsung is the big loser here, falling to 4.5 percent usage worldwide and 7.3 percent in the US.

Windows Phone almost hits 50 percent usage. This month, Windows Phone 8 barely missed hitting 50 percent usage. We can expect that milestone to happen in June, given that Windows Phone 8 did gain over 6 percentage points of usage month over the month.

Hints of ecosystem growth. As AdDuplex notes, it only measures Windows Phone usage, so it can’t compare its data to that of iPhone or Android users. But it has seen evidence of Windows Phone ecosystem growth: The average numbers of unique users of Windows Phone apps participating in the AdDuplex cross-promotion network has grown near linearly since January 2012. “The average numbers of unique users in Windows Phone apps participating in the AdDuplex cross-promotion network has grown about three times over the 2012 average,” the firm notes, “which isn’t bad at all.”

New unidentified devices. As is often the case,AdDuplex notes the arrival of some mysterious new Windows Phone handsets in this month’s report, in addition to frequent sightings of the Nokia RM-(Lumia 928), due for imminent public release. New hardware this month includes the SAMSUNG SPH-I800 (720p screen, most likely a version of ATIV S), Nokia RM-941 (WVGA display), Nokia RM-910 (WXGA), and Nokia RM-875 (also WXGA).

First sightings of Windows Phone 8.1. AdDuplex also reports “quite a few phones running Windows Phone 8.1,” the upcoming next version of Windows Phone. These are mostly Nokia P4301 test devices, but there are also some HTC 8X and Lumia 920s in there too. AdDuplex reports the build numbers as being in the 8.10.12072 to 8.10.12079.0 range. By comparison, my Windows Phone 8-based Lumia 920 is running build 8.0.10211.204.