Observation - $lastn

I am a little confused on how the description is laid out for the usage of $lastn

Here is an example -

I could have 4 observations -
Height on 2017 Apr 25 - 5 ft 2 inches
Body Height on 2018 Aug 27 - 5 ft 3 inches
Body Height on 2019 Apr 25 - 5 ft 4 inches
Height on 2019 Apr 26 - 5 ft 5 inches

When I do a search with filtering values as Body Height, Height what would my result look like in the response, in this case the consuming application would have to decipher this information to pick the right value to show?

1 record for Height
1 record from Body Height

I am expecting only 1 result value the latest with Height on 2019 Apr 26 5ft 5 inches.

Please review and advise!

Any guidance would be much appreciated!