Here is a list of known Adaptation Kit Update (AKU) releases and their associated build number. Betaversions are not listed. WhatIsAnAKU?

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Windows Mobile 6.5

AKU Release	Build	Comments
5.0.0
5.0.1	21234	Emulator images

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Windows Mobile 6.1

AKU Release	Build	Comments
1.0.0	19202	Emulator images
1.0.2	19209
1.2.3	19965
1.3.3	20273
1.4.0	20757	Emulator images
1.4.3	20264
1.4.4	20765
1.4.5	20769
1.5.0	20954
1.6.0	21014
1.6.1	21042

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Windows Mobile 6

AKU Release	Build	Comments
0	15342
0.2	17740
0.2.1	17741
0.2.2	17742
0.2.4	17746
0.3.1	17944
0.4.2	18125
0.4.4	18128
0.4.5	18129
0.7.0	18541
0.7.4	18550

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Windows Mobile 5.0

AKU Release	Build	Comments
0	14343
0.1	14352
0.1.1	14354
0.1.2	14355
0.2	14359
1.0	14363
1.0.1	14366
1.0.2	14367
1.1	14402
1.1.1	14406
1.1.2	14409
1.1.3	14410
1.1.4	14411
1.3	14523
2.0	14847	MSFP
2.1	14914
2.2	14928
2.2.1	14929
2.3	14955
2.3.2	14959
2.5	14967
3.0	15096
3.0.1	15097
3.2	15633
3.3	15671
3.3.1	15673

The MSFP (Messaging and Security Feature Pack) was included in Windows Mobile 5.0 AKU 2 and later.
See this section on MSDN for details on MSFP.

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You can obtain the build number and AKU of a particular device in several ways:

From the device itself

When using the device, type Start / Settings / System Tab / About
The information displayed includes a Build number, for example Build 14967.1.1.
The larger number is the build number, and from that, you can determine the AKU release.


Programmatically

Using a C# application, the Environment.OSVersion.Version method returns build information.


Programmatically, Using the Registry

The AKU version is stored in the registry: HKLM\SYSTEM\Versions\Aku


What was added in each AKU?

Ideally, as a developer you don't need to worry about this. The goal of Windows Mobile is to be a stable platform, to ensure that any application written for a Windows Mobile device will run on any other Windows Mobile device. Still, this is interesting: Review of update packs