As this gauge is not UK but used on the east coast of USA you might do better to ask on the HSM forum.

I thought it was 1 1/16 in/ft. This scales 4' 8.5" to 5.002" which is near enough for most people. It is of course hellish inconvenient. 2.25 mm to the inch is also pretty accurate, and a lot handier (unless you're a diehard Imperialist)

ME 2590 (1951)

The 3/4’;-in. Gauge
Q SEVERAL READERS
have noticed that in
the recent note
(December 7th issue)
about the new track
belonging to the
Golden . Gate Live
Steamers, of Oakland,
California, U.S.A., the
available gauges are
stated to be 2 1/2-in.,
3 1/2-in. and 4 3/4-in. and
we are asked if we can
explain why the 5-in.
gauge is, presumably,
not recognised by our
American friends.
This is an old story
, which was fully debated
some 2o years ago
when the 5-in. gauge
¿rst came into use as a substitute for 4 3/4-in. gauge
in Britain. The controversy centred round the
then rapidly-growing desire to see true-to-scale
locomotives built to I-in. scale, but capable of
giving trouble-free and lasting service as passen-
ger-haulers, and matters came to a head when
interest was directed towards old-time types for
the purpose stated. One inch to the foot, is
apparently, a delightfully simple unit to use,
and 4 3/4 in. is a clear and ¿nite dimension to
which track can be laid; but many ‘people
have been surprised to discover how few British
locomotive types can be reduced to these
dimensions and give, at the same time, true-
to-scale appearance, robust working parts and
essential working clearances We all know now
that the problem was solved by the adopt1on_of
5-in gauge and 1 1/6 in scale, a combination which
gives a more practical scale/gauge ratio to which
almost any type of locomotive can be constructed
to give satisfactory service and very nearly
exact scale appearance
The American locomotive does not present
the same problems, due to the fact that its
dimensions are larger, in all directions, than those
of ours , so our American friends have never
had any reason to ¿nd fault with the 1-in scale on
4 3/4-in gauge The fact that the gauge is very
slightly wide for the scale does not affect a
mimature American locomotive to anything
approaching the same extent as it does a
British one, especially an old timer For
example, the Brighton Gladstone Class
would be very difficult to reproduce 1-in.
scale on 4 3/4-in gauge , but, as everybody
now knows, it is easily possible in 1 1/16 scale
on 5 in gauge

Edited By Ady1 on 23/08/2023 23:03:47

Not too sure if you have seen it but there is a old 43/4” gauge loco in the Station Road Steam archive. Search for “antique tender engine”. Might be or interest.