Agatha Christie question: A Murder is Announced

This question isn’t really plot related, but I’ll use a spoiler box just in case:

In the book, one of the characters (Sonia Stamfordis) gives birth to twins. The births occur just after noon. As a result, she names her two children Pip and Emma. From the context these names appear to be some sort of joke, although it’s not explained why the names are so humorous.

Can anyone elucidate?

Pee Em
PM
Post Meridian.

Oye.

Yes, I’d thought of the *post meridiem * link, but the wording in the book seems to imply more of “in” joke than that.

It was my belief that either the British military or the BBC or both used “ack emma” for the a.m. and “pip emma” for the p.m. There was no hidden “in” joke. Any Britisher would’ve gotten the joke, such as it was. Dame Agatha fortunately had a second career to fall back onto when her stand-up comedy career didn’t work out for her.*

*yes, I know her back up career was as an amateur anthropologist.

Ah, that sounds more like it. Thanks.

That’s right – the British Army phonetic alphabet from 1927 was:
Ack Beer Charlie Don Edward Freddy George Harry Ink Johnnie King London Monkey Nuts Orange Pip Queen Robert Sugar Toc Uncle Vic William X-ray Yorker Zebra.

“Ack”, “Emma” and “Pip” had been in use since at least 1904, and apparently the OED has a reference for Beer and Emma dated 1891.