The prototype chassis carried an X here, i.e. VRX), xxx reflects the chassis details, and yyy the engine, e.g. VRT/SL3/6LXB is a short, low, Series 3 with a transverse Gardner 6LXB engine.
ðə pɹˈoʊɾoʊtˌaɪp tʃˈæsi kˈæɹid ɐn ˈɛks hˈɪɹ, ˈaɪ.ˈiː. vˌiːˌɑːɹɹˈɛks, ˌɛksˌɛksˈɛks ɹᵻflˈɛkts ðə tʃˈæsi diːtˈeɪlz, ænd jjˈaɪ ðɪ ˈɛndʒɪn, ˈiː.dʒˈiː. vˌiːˌɑːɹtˈiː slˈæʃ ˌɛsˈɛl θɹˈiː slˈæʃ sˈɪks ˌɛlˌɛksbˈiː ɪz ɐ ʃˈɔːɹt, lˈoʊ, sˈɪɹiz θɹˈiː wɪð ɐ tɹænsvˈɜːs ɡˈɑːɹdnɚ sˈɪks ˌɛlˌɛksbˈiː ˈɛndʒɪn.