I've done a little digging and there are two versions of the 'chrome bumper' design. The first debut version is known as the 'bolt together' version, as the top, bottom and side panels were separate, with the front edges displaying a raw aluminium 'chrome' finish.

Later versions had a single extruded aluminium outer sleeve, which is what I meant by the 'wrap around sleeve', again with the front edge having the 'chrome' finish.

Internally there were differences in the layout and design of the PCBs, as well as how the output transistors were heat sinked. Also the early versions used Cannon LNE mains connectors.

I have no real experience of these Naim 250 amplifiers apart from listening to a couple at two separate audio dealers. My interest at that time was to audition solid state amplifiers that could happily drive Quad ESL-57 speakers.