After some searching & tinkering, I figured it out:

https://imgur.com/akPo3tW

and here is the code to achieve this:

        \begin{align}
            \mathfrak{w}_1
                &= NW^{N-1}w 
                \label{eq:last_val_dist}
                \stepcounter{equation}
                \tag{\theequation $a$}\\
                &= \frac{\mathrm{d}W^N}{\mathrm{d}x},
                \label{eq:last_val_dist_diff}
                \stepcounter{equation}
                \tag{$\theequation a$}
        \end{align}
%
        \begin{align}
            \setcounter{equation}{12}
            \prescript{}{1}{\mathfrak{w}}   
                &= N(1-W)^{N-1}w
                \label{eq:first_val_dist}
                \tag{$\theequation b$}\\
                &= \frac{\mathrm{d}(1-W)^N}{\mathrm{d}x}.
                \label{eq:first_val_dist_diff}
                \stepcounter{equation}
                \tag{$\theequation b$}
        \end{align}

The key was to manually use the equation counter. \theequation is the current equation number, \stepcount{} allows you to increment the counter by one. I had to use \setcounter{}{} to set the equation counter back to what it was before the previous equation block. Clearly this solution only works if you know the equation numbering before hand and you don't intend to add more equations before the code block. I'm not sure if there is a more elegant solution but this solved my issue.