"We went swimming later in the afternoon, Jack and I."

I am trying to describe what is happening here by breaking the sentence down into it's basic components, but I am having difficulty doing this. The "jack and I" part is the Noun Phrase. right? Is there a certain language formality to placing this at the end of the sentence behind the comma? Am I crazy in thinking that the above format sounds more formal than: Jack and I went swimming later in the afternoon.

I'm adding some more information that has come up through the conversation below:

I pulled the line from a Young Adult book written in 1942. I'm studying the text and trying to identify elements that make it 'feel old.' One of these elements is a general presentation of phrases in a more formal way (as compared to other modern YA publications.)

Why does the following phrase sound old fashioned?

We went swimming later in the afternoon, Jack and I.

I am trying to describe what is happening here by breaking the sentence down into its basic components, but I am having difficulty doing this. The "Jack and I" part is the Noun Phrase, right? Is there a certain language formality to placing this at the end of the sentence behind the comma? Am I crazy in thinking that the above format sounds more formal than "Jack and I went swimming later in the afternoon"?

I'm adding some more information that has come up through the conversation below:

I pulled the line from a young adult book written in 1942. I'm studying the text and trying to identify elements that make it 'feel old.' One of these elements is a general presentation of phrases in a more formal way (as compared to other modern YA publications.)

"We went swimming later in the afternoon, Jack and I."

I am trying to describe what is happening here by breaking the sentence down into it's basic components, but I am having difficulty doing this. The "jack and I" part is the Noun Phrase. right? Is there a certain language formality to placing this at the end of the sentence behind the comma?

"We went swimming later in the afternoon, Jack and I."

I am trying to describe what is happening here by breaking the sentence down into it's basic components, but I am having difficulty doing this. The "jack and I" part is the Noun Phrase. right? Is there a certain language formality to placing this at the end of the sentence behind the comma? Am I crazy in thinking that the above format sounds more formal than: Jack and I went swimming later in the afternoon.

I'm adding some more information that has come up through the conversation below:

I pulled the line from a Young Adult book written in 1942. I'm studying the text and trying to identify elements that make it 'feel old.' One of these elements is a general presentation of phrases in a more formal way (as compared to other modern YA publications.)