Great Escape Games 12mm 1914

 This range looks interesting. Here. Has Belgians including artillery shown.

Zouaves

Van Dyck 12mm WW1

https://vandyckmodelsandfigurines.be/WP/product-category/ww1/

Belgian Minerva

I am hoping they fit in with Kallistra

Kallistra 1914

 These figures look good. 12mm. Watch video here

They also have rules Raging Empires for free download here

Van Dyck 12mm World war one

http://vandyckmodelsandfigurines.be/index.php/furniture/world-war-one.html

A range of Belgians and Germans that look very good. And they do a Minerva armoured car.

Kallistra 12mm Early WW1

 I wonder if these are compatible? https://www.kallistra.co.uk/?page=113

Belgian Dragoons 1914 by Kallistra

1914

 Been looking for a rule set that specifically deals with 1914 and found one. Created for 12mm figs that they produce. There is a sample Pdf of the rules and pictures of the figures. Anyone got this? I must say it looks interesting.

https://www.greatescapegames.co.uk/1914

Kallistra Early World War One 12mm

There are some decent figures out there in this scale.  Website here

Image: 1/1 - Belgian Early War Field Gun Teams and Limbers
(2 teams + bases)
WW1-BG06

1914 - from Great Escape Games

Designed with 12mm figures in mind this rule set looks good. They do a range of figures available through North Star
Some photos of a game in progress at this blog

Van Dyck Models 1914 Germans 12mm

1/144th scale.  Really well-detailed sculpts these. Already has a great range of Belgians. Germans here
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They have some photos of a forthcoming armoured Belgian train that looks good. 

Van Dyck: 12mm Belgian Line infantry and Minerva Armored Car

go here to see what looks like a very good range.See more here