The UK and Ireland censuses are available from multiple places: Ancestry, FindMyPast, and MyHeritage to name a few. FamilySearch has transcripts, but it links to FindMyPast for the images.
I can access the image at Ancestry and MyHeritage through my library. In both of these, the image is not very clear. However, on MyHeritage, I was able to zoom large enough so that I think we can confirm that it is a lowercase A with an underscore. In the zoomed image below, you can see that the character has a tail on the right side, and you can barely make out a light spot in the center.
The previous page has people living at 59 and then 3U (3 uninhabited buildings). 61a may be half of a duplex with 61b (which is not listed), or it may be a garage apartment or other dwelling behind 61 proper, which is one of the 3U between 59 and 61a. It would be helpful to locate a city directory for this time period to see how the house numberings go.
The ages and birth places for this household are listed as follows:
||Calculated Birth Year
||1852 or 1853
||1877 or 1878
||1879 or 1880
||1881 or 1882
||1885 or 1886
||1886 or 1887
||1888 or 1889
||1889 or 1890
South Molton is the city of the current (1891) page.
This page lists Mary Ann's marital status as M but where's William Sr.? Also, the daughter who is 5 may be the one that was born in 1885, but this one is listed as having been born in Barnstaple, so either that is oddly specifically incorrect (she's the only Barnstaple), or there is a missing child, or this is a different family entirely.
Directories (parish, city, county) for the corresponding places and years from the table above may be helpful if you can find them.