As LE Greys has mentioned, about five HST TGS vehicles were modified with buffers at one end to allow class 91s to be attached to that end of the mark 3 formation in the early days of testing and operation of the 91s on the ECML, before enough mk 4s became available. The HST surrogate DVT was at the south end coupled to first class. Once the operation of 91s + mk3s was over, these five TGS vehicles were released into general use in the HST fleet, still with buffers, and IIRC ended up split between East Coast and Crosscountry operations. I'll see if I can dig out the numbers later.
Intercity train formations in the UK are filled with discrepancies with missing coach letters. In the case of Grand Central, it seems that the Coach A designation has been retained for any possible and currently unintended insertion of a TGS into the formation in the distant future. A bit peculiar, but seeing as both the HSTs and 180s are five carriage trains, it therefore makes sense for Grand Central to arrange the carriage lettering for the 180s in the same way to avoid confusion, seeing as Grand Central 180s and HSTs share diagrams on the Sunderland runs.
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There seems to be one set permanently formed like that. I'm not sure about this instance but for the past two years it's hasn't been due to an extra carriage being added to the rake, it's because there aren't sufficient TSOs available.
