[StainlessS]

The script is using a 8 bit mask over a 16 bit base clip. Since no automatic bit-depth conversion happens, the result is undefined.

The fact that it worked at all, made me think that it was supposed to work [8 bit Overlay normally works with any colorspace].
Kudos that it worked at all and did not go BANG!

# ...

Something like this (I've used PlusBuildNumber, maybe version number should return the 3.4 bit, dont know)

Function IsAvs26()                  { VersionNumber>=2.6}
Function IsAvsNeo()                 { FindStr(VersionString," Neo")!=0}
Function IsAvsPlus()                { FindStr(VersionString,"AviSynth+")!=0||IsAvsNeo}
Function PlusBuildNumber()          { V=VersionString Off=(!IsAvsPlus)?0:FindStr(V,"(r") return (Off==0)?0:V.MidStr(Off+2).Value.Int }  # OK Avs+ & Neo, up to build about 16Million
Function AvsPlusVersionNumber()     { Return PlusBuildNumber }  # Stub for PlusBuildNumber(), suggest AvsPlusVersionNumber is deprecated.

ADDED

EDIT: Eval("2983,") would fail due to trailing comma, "2983,".Value.Int, Value ignores comma but produces float, Int converts to integer. [supports more than 4 digits, max 24 bit, ~16M]

EDIT: Version Strings

# AviSynth v2.58
AviSynth 2.58, build:Dec 22 2008 [08:46:51]

# AviSynth v2.60
AviSynth 2.60, build:Mar 31 2015 [16:38:54]

# AviSynth v2.61 Alpha
AviSynth 2.61, build:May 17 2016 [16:06:18] VC2008Exp

# Avisynth v2.60 ICL
AviSynth 2.60 (ICL10)

# Avisynth V2.60 MT
AviSynth 2.60, build:Feb 20 2015 [03:16:45]

# Avisynth NEO
AviSynth Neo 0.1 (r2822, Neo, i386)
AviSynth Neo 0.1 (r2822, Neo, x86_64)

# Avisynth NEO Forerunner Avisynth+ CUDA
AviSynth+ 0.1 (r2533, CUDA, i386)
AviSynth+ 0.1 (r2533, CUDA, x86_64)


# Avisynth+ v2.60 (older)
AviSynth+ 0.1 (r2772, MT, i386)
AviSynth+ 0.1 (r2772, MT, x86_64)

# Avisynth+ v3.4 (Prev)
AviSynth+ 3.4 (r2923, 3.4, i386)
AviSynth+ 3.4 (r2923, 3.4, x86_64)

# Avisynth+ v3.4.1 (NOW)
AviSynth+ 3.4 (r2983, linux3, i386)
AviSynth+ 3.4 (r2983, linux3, x86_64)

So, VersionNumber is 2.60, PlusBuildNumber is 2983 [ Previously known as AvsPlusVersionNumber(), see above in BLUE] , what do we call the 3.4.1 stuff ?
[ Something_MAJOR, Something_MINOR, something_BUGFIX ??? ]