OH2.5 offline add-on install problem

OH 2.5.12 , Java 8, Windows host - which might be relevant.

I’ve an openHAB 2.5 installation (yes, still) happily running in a commercial setting, and want to add SNMP binding to talk to a replacement UPS.
I’m well aware of Bintray repository changes-
https://community.openhab.org/t/openhab-2-5-x-and-the-sunset-of-bintray/121839
and had previously elected to download the addons kar bundles, so that ‘offline’ installation should take place.
All good so far.

However, attempting install of SNMP v2 binding gives openhab.log error

2021-11-24 20:47:13.902 [ERROR] [core.karaf.internal.FeatureInstaller] - Failed installing 'openhab-binding-snmp': Error:
	Error downloading wrap:file:/C:/openhab2/userdata/tmp/kar/openhab-addons-2.5.12/javax/activation/activation/1.1.1/activation-1.1.1.jar$Bundle-Name=Javax%20Activation&Bundle-SymbolicName=javax.activation.activation&Bundle-Version=1.1.1
	Error downloading wrap:file:/C:/openhab2/userdata/tmp/kar/openhab-addons-2.5.12/javax/activation/activation/1.1.1/activation-1.1.1.jar$Bundle-Name=Javax%20Activation&Bundle-SymbolicName=javax.activation.activation&Bundle-Version=1.1.1

Note the error is about javax activation-1.1.1.jar ,not snmp binding directly.
I guess it is a dependency, I know little of these things. Looks to be a MIME handling utility.

I checked in folder
C:/openhab2/userdata/tmp/kar/openhab-addons-2.5.12/javax/activation/activation/1.1.1/activation-1.1.1.jar
and yep, the addons kar has been successfully unpacked and includes that file.

The bit worrying me is the
Error downloading wrap:file:/C:/openhab2/userdata ... etc
is that wrap: an ordinary thing here?
I ask because I saw a longer dump on the console includes
Caused by: java.net.MalformedURLException: unknown protocol: wrap

Can I work around this by installing the package? How would I do that, I’ve no idea! (bearing in mind Windows hosting)