Why do some pulsar planets have designations with a capital letter at the end?
I will just cite this from Wikipedia:

On their discovery, the planets were designated PSR 1257+12 A, B, and C, ordered by increasing distance. They were discovered before the convention that extrasolar planets receive designations consisting of the star's name followed by lower-case Roman letters starting from &quot;b&quot;, in order of discovery, was established. However, they are listed under the latter convention on astronomical databases such as SIMBAD and the Extrasolar Planets Encyclopaedia, with A becoming b, B becoming c, and C becoming d.

In a nutshell, this is the older nomenclature still kept because of historical reasons.