Try this:
```
<$list filter="[<currentTiddler>fields[]prefix[group]]" variable="thisgroup">
   <$view field=<<thisgroup>>/><br>
   <$let itemprefix={{{ [[item-]] [<thisgroup>split[-]nth[2]] +[join[]] }}}>
   <$list filter="[<currentTiddler>fields[]prefix<itemprefix>]" variable="thisitem">
      &emsp; <$view field=<<thisitem>>/><br>
   </$list>
   </$let>
</$list>
```
Notes:
* Inside the outer `$list` widget, we construct the desired "item-#" prefix by using a "filtered transclusion" to join together the literal text "item-" and the number for the current group, which we extract from the `thisgroup` variable using `split[-]` and `nth[2]` filter operators.
* We can then use the constructed prefix in the second `$list` widget's filter.
* I added an `&emsp;` (a full em-space HTML character) to provide indentation for the item output

enjoy,
-e