I found a video of this variant; the turret reminds me of the French AML-90’s, but it’s different. It’s not the Brazilian turret either, it might be a Chilean one.

As a side note, the source also allegedly depicts a PT-76 fitted with a Cockerill Mk. III gun.
Source:
CMI (Cockerill) brochure

CMI Defence. (2000). Cockerill Mk3/CSE 90 mm weapon system [Brochure]. Internet Archive. https://web.archive.org/web/20070331063456/http://cmigroupe-web.com.acelis.net/files/files/defence/cse90.pdf
CMI Defence included the PT-76 among the vehicles covered by its Weapon Systems Modernisation Programmes . The programme presented a range of potential upgrades intended to improve the performance of existing weapon systems.
For the PT-76 specifically, the brochure lists general turret reconditioning , replacement sights , direct day and optional thermal sights , I² night vision , a laser rangefinder , and, most notably, the replacement of the existing gun with a Cockerill Mk3 gun .
The table also indicates that CMI offered this gun replacement for the PT-76 alongside a similar upgrade for the Alvis Scorpion. The brochure does not, however, provide further details on the exact PT-76 configuration, ammunition, or whether a Cockerill Mk3-equipped PT-76 was actually produced.

Source: CMI Defence. (n.d.). Cockerill Mk3/CSE 90 mm weapon system / Cockerill Mk8/LCTS 90 mm weapon system [Brochure]. Internet Archive. https://web.archive.org/web/20070331184023/http://cmigroupe-web.com.acelis.net/files/files/defence/lcts90.pdf

