Once Belgian production began, there were real not just cosmetic differences between the two factories products.
Variations
One of the things which differentiated French Bertins from other home market semi and mass producers of bicycles was their commitment to quality framesets. Only the very lowest price points used hi-tensile steel for the frames and forks. All the other French models, from just above basic to top-of-the-line proudly wore the Vitus, Reynolds or Columbus stickers which announced the frameset’s pedigree.
One difference with Belgian Bertins was, that for much of the production of the Belgian factory, the frames only infrequently wore tubing decals identifying the steel beneath the paint.
Original Factory – Paint Finishing Area
New Factory Automated Paint Line
As well, the frame covering often wasn’t even paint as the Belgian production framesets were powder coated, one of the first commercial applications of the technology on “mass-produced” bicycles I have ever seen.
Production in France was wet paint technology as can be seen in the two accompanying photos that often serve as rotating header pictures. The earliest shows the original factory’s spray shop and the other shows the automated production set up of the “new” factory built after 1973 in St. Laurent-Blagny.
Powder coating was invented in the late 1940s but really became widely commercially available in the mid-1960s. It seems likely that Belgian production dates from that period. The smaller volume in Poperinge may have permitted the alternate method whereas higher production volumes and more elaborate paint finishes in France may have necessitated the fast dry capability of wet applied paints. Typically, the Belgian Bertins with powder coat seem to be single colour in application due to the difficulty of masking and baking on of various colours.
Another distinction between French and Belgian production was equipment. French Bertins wore all French, Milremo, Campagnolo and, of course, Shimano equipment. Cycles produced in Poperinge had Weinmann brakes in lieu of MAFAC or CLB, Weinmann rims, Sakae cotterless cranksets, Ishiwata tubes and varying types of seatposts and stems. All of this leads me to infer that the Belgian produced equipment was specified to avoid tariffs on French products because the branding is consistent across models and price points.
Finishes
Jan Janssen with Half-Paint Socks
Belgian Bertin with Half-Painted Socks
The finishes on Belgian produced Bertins didn’t conform to the French ones but they did adhere to a unique Belgian esthetic.
The following photo is of a Janssen – another Belgian producer which has the distinctive socks on the forks and stays often found on 60s and 70s bikes. Except these are painted silver and not the more typical chrome found just about everywhere else. And Bertins conformed to this practice as can be seen in the next sample of a comparable pale blue Bertin model found to the left.
As well, Bertins had contrasting paint applied to their head tube lugs or had the head tube and lugs painted a contrasting colour uniformly. Typically, a French Bertin would have only the space between the head tube lugs contrasting.
Bertin paint finishes in Belgium often had finely detailed pinstripes on the tubes which their French cousins lacked altogether or had in much lesser abundance. Some pinstriping was distinctive like the “coffee bean” type found on this older model of Belgian 
Bertin located on either side of the down tube lettering.
Another distinctive were the racks attached to the sport, utility and touring varieties of Belgian production. The Cycles Bertin method 
was to build chromed steel racks and then attach them firmly to frame braze-ons or, occasionally, to fenders as well.
Belgian production brazed the racks directly to the rear stays and dropouts and painted them the frameset colour making for an outstandingly robust rack but with a parallel sacrifice of flexibility in the use of the cycle.
Like their French cousins, Belgian production often decorated the caps of the rear stays with contrasting paint or decals. However, they also had multi-coloured accent treatments on the fork crowns as well with up to three colours on occassion as well as paint on the lugs, a graphic treatment abandoned in France in the late 60s or early 70s.
Decals
Decals in Belgian production had a quirky non-conformity to them. The bikes were produced under licence and used the same foil head tube decal so familiar from the French Bertins produced in St. Laurent-Blagny. Very early Belgian production, typically wore the A.BERTIN decal seen in the photo above, which in France typically showed up on utility and folding bikes. As well, Belgian bikes wore French tri-colour ribands around their 
seat tubes rather than the red/yellow/black tri-colour of Belgium or the World Championship stripes seen on French production.
The later BERTIN decals seen on Belgian Bertins were completely different sans-serif capital letters unlike any of the graphics from France. Similar? Yes, but unique. (See the Decals section in Part 3 for samples.) Sometimes, the seat mast decal would have a solid colour and a second BERTIN label would be found on the down tube. Although similar to the French style and placements, Belgian Bertins had their own unique style.
Bertin Classic Cycles 