This stem was available in both Reynolds 531 steel (as per this example) and in Accles and Pollock 'KROMO' steel. Both high-tensile steels, the former being much more famous.
The price was the same in either material. From the surviving GB catalogues, the Tony Merken only appears in the early 60s one. It was available in 31/8", 3¾" & 4" extension lengths. This was the first GB stem to have a distinctive model name. Unfortunately, GB seem to have got it slightly wrong since the champion German track rider after whom it is named (and who can be seen using this design of stem in contemporary photos - even if it is not an actual GB one) was called Merkens with an 's' at the end!