Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

The obvious difference is that the 105 doesn´t have a whistle (or a hole for it in the boiler), whereas the 105/1 has.

Other less obvious difference would be the size, as the 105 is quite a bit smaller, standing 2 inches lover than its sibling, on a slightly smaller base, with a 48mmx65mm boiler, compared to one measuring 52mmx81mm on the 105/1.