The difference between the Oral-B iO5 Duo with the product code "4210201415121" and the Oral-B iO5 Duo Black UCB/ White SC (414834) lies mainly in the following details:

## Colours and Design
- The Oral-B iO5 Duo with product code "4210201415121" may be available in different colours, but exact specifications are not provided. However, model "414834" is specifically mentioned as being available in Black UCB and White SC.

## Features and Accessories
- Both models have the same basic features, including 5 cleaning modes (daily clean, sensitive, super sensitive, intense, and whitening), cleaning mode icons on the handle, and a reminder to replace the brush head via the cleaning mode button.
- There's no significant difference in the accessories included, as all iO5 models generally come with a charging station and travel case.

## Price
- The price quoted for model "414834" is 178.85, but the price of the model with product code "4210201415121" is not specified. However, prices for iO5 models are generally similar, with minor variations by region and seller.

In summary, the main difference is the colours available and potentially the price, but the features and accessories are essentially the same.