For that - and to replace the current T-20 Chiliean-acquired Carro de Combate Modelo 1941 tanks; Belgium is issuing a requirement for 340 new medium tanks for delivery from 1950 (potentially another 170 may follow later in the early 1950s to replace the T-21 (Cromwell).
Specs are loose but it should not be too heavy and possess good cross-country performance. Good armour against the latest tank-guns is required.
The current armament of the 75mm L/54 FRC is getting a bit long in the tooth so the 90mm L/60 Cockerill (as used in the Dutch T-44) is specified but given the size of the order and the chance to standardise on a new gun, again options are open if manufacturers can provide a better weapon and provide good arguments to use it.
France
The French can offer the current-production
AMX-40B3 Tigre, and it should meet or exceed pretty much everything the Belgians might demand of it. The current French Army ideal is for very fluid fire-and-maneuver warfare, and the -40 is designed to fit that role: a high level of attention has been paid to crew awareness and comfort / fatigue reduction, main gun stabilization, light (well, for a tank) overall weight, and good mobility. Most of the mechanical quirks have been worked out of the design by the AMX-40B3 variant, and the French have over a thousand currently in service, so definitely no risk to long-term technical support or limited spare parts.
The version currently manufactured for the Armee de Terre has the French CN-90 Mle.42 m.46 F2 gun (90mm/L64), so the Cockerill 90mm/L60 should drop in quite nicely.
Russia
The Russian Federation, in turn, can offer the
T-47 Tsiklon, which does all of those same things for the Motherland. Obviously, it is
stronk Russian tenk, although the T-47 does, of course, make a few minor tradeoffs for its various strengths. That said... the D10T 100mm/L55 gun is one of the largest and most powerful tank guns in service at the moment. Did I mention the price is excellent? The Russians are always great with economies of scale, and by the end of 1949, the Russian Ground Forces will have... hm, let's see, add these, carry the five... yes, seventy-five hundred of them in service.
Na Rodina!
If the Belgian politicians are looking for the lowest bidder, then the Russians can also offer the T-45 Grom, which is the ultimate development of the earlier T-44 medium tank. It also has the D10T 100mm gun, but manages it on a 36-tonne chassis. Only used by the Russian Naval Infantry (and sold to the Philippines), but it's even cheaper than the Tsiklon. There are, ahem, a few less creature comforts.
NPCs
Skoda (Czechoslovakia) can offer the
Skoda TVP vz.47 Tank for General Use (scroll down to the second half of the post). A Cockerill 90mm gun should be able to drop in without much trouble.
As a side note, I'd intended for the Czechs to develop a follow-on vehicle to the vz.47 which would enter production in mid to late 1950. I've never written down any specs for this vehicle, but it would be a heavily-improved variant of the vz.47. The original vz.47 has rather mediocre frontal armour sloping, and the new vehicle will fix this. Designation will probably be something like
Skoda TVP vz.50. If you're interested, I'll do a writeup and polish out the rest of the specs. Just keep in mind that the first deliveries to the Czechs will probably be July 1950, so if the Belgians want most of the order delivered in 1950, this might not be an ideal choice.
The Polish
32TP is, of course, always an option; but the 90mm Cockerill gun will probably stress the turret, and the armour's not really best-in-class for 1950. The Polish have a new design in the works, but I'd intended to leave that for post-1950.
The Italian
M37/45 Princeps should probably be regarded as a contender as well, even if Italy is NPC.