The 391st has a website and a page which mentions the 873rd several times.
https://www.391bombgroup.org.uk/53SvGpHist.php
If you search for 873 on the page you find several entries related to their duties.
For example, one entry states:
"In addition to their regular duties of handling and storing Chemical Ammunition, these men assisted the Service Group Ordnance Company and the Ordnance Sections of the Bomb Squadrons in handling and loading planes with Ordnance Ammunition."
Another series of entries for June 1944 mentions duties as follows:
2. The organization performed usual camp duties for the month of June, which consist of the following:
a. Regular inspections to determine any discrepancies in the organization
b. Storing, handling, and shipping of Chemical Ammunition.
c. Testing and handling of leaking Mustard Bombs.
d. Assisted in the loading of planes to perform missions.