This post is to highlight some of the possibly lesser collected postmarks - the ones often applied to the reverse of envelopes. These were applied either when mail was sent (perhaps because the cancelling postmark did not have a date), or when it transitted an office, or when it was received. Similar postmarks were also applied to forms and receipts, for example for registered post.
The postmarks are often very similar, small single rings with the name of the post office. I've found that the only reliable way to distinguish them is to scan the postmarks and overlay them to check the geometry - how tall/short the letters are, how far round the cancel the lettering stretches, and where the lettering is positioned in relationship to the date and rim.
For comparison, a limited number of the postmarks are available in the Stamp Impression Books where they were registered on being issued, but unfortunately these are not all available as many were lost in a fire.
Here is a page of Ilminster single ring handstamps.