How did someone modify the WordPress "page editor" page?

On my client's WordPress website, the original developer added a drop-down field to the page editor. See the image below:
 Page editor showing custom featureI need to modify the drop down, and can't find it anywhere (I downloaded the entire theme and searched all the files).
Where would this be stored? How can I modify it so that instead of 5 items in the dropdown there are simply 6?

Thank youl.
Lev SeltzerAsked:
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If it's a custom theme, probably no one here can find it unless you upload the full theme.
Lev SeltzerAuthor Commented:
Wouldn't a custom theme mean that there is a file in the "/themes/custom_theme_name" folder that contains the text that I see on the "page edit" page?
Because if so, I download the files and looked through them with a program and could not find this code.
Some contents are fully saved in the database, relying only on a generic template file.
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Lev SeltzerAuthor Commented:
Good thinking. So I did some searching and found the field in the post_meta table, and a reference to this field in a template:
get_field( "choose_left_sidebar_donate_widget" );

But this data is connected to the actual record, as opposed to the page setup. Where are reasonable locations to look for the location that controls this field on the "page edit" page?

Below is the <div> that contains this dropdown. No code here because this is from the browser. But perhaps you might see a clue here?

<div id="acf-choose_left_sidebar_donate_widget" class="field field_type-select field_key-field_5402e39288e04" data-field_key="field_5402e39288e04" data-field_name="choose_left_sidebar_donate_widget" data-field_type="select">
	<p class="label"><label for="acf-field-choose_left_sidebar_donate_widget">Choose 
	Left Sidebar Donate widget</label></p>
	<select id="acf-field-choose_left_sidebar_donate_widget" class="select" name="fields[field_5402e39288e04]">
	<option selected="selected" value="1">1</option>
	<option value="2">2</option>
	<option value="3">3</option>
	<option value="4">4</option>
	<option value="5">5</option>

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Jason C. LevineDon't talk to me.Commented:
<div id="acf-choose

ACF is Advanced Custom Fields, which is a plugin.  Look for that in your admin backed.

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Lev SeltzerAuthor Commented:
Thank you both for helping me to find this. In fact, the programmer did use Advanced Custom Fields 4.4.2, and I was able to find this field, along with the page to modify the drop-down options.
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