Not sure where I found it online, but thanks to the original poster.
Dating
Sun Cycles from frame numbers.
Pre
1934 a frame number only code was used, found either at the lower
front of the head tube or nearside of the seat lug. Sun factory
records being destroyed this code is not yet positively broken at the
time of writing. To date all verified Suns carry either an eight, six
or five figure number. However, from earliest days, Brown Brothers,
who were the largest wholesalers of Sun fittings, produced frames and
complete cycles indistinguishable from factory originals (compare
1902 Brown Brothers and 1905 Sun frame illustrations) except fot the
frame numbers. As Sun also sold complete frames and cycles to small
shops to affix their own transfers up to the early 30s identifying
early period Suns without transfers can be difficult.
From
1930 a rolling letter code similar to their motorcycle system was
adopted whereby the first letter of the code refers to the financial
year running from 1st
April to 31st
March the following year. This is set out here:
A1934/5 B35/36 C36/37 D36/37
E38/39 F39/40
G40/41 H41/42 J42/43 K43/44 L44/45 `M45/46
N46/47 O47/48 P48/49 Q49/50 R50/51 S51/52
T52/53 U53/54 V54/55 W55/56 X56/57 Y57/58
Z58/59
Second
(or last if there are three) letters refer to frame building shops
and appear as one of the following: C/D/E/F/H or P/R/S/T/V/W. Parkes
lightweights usually carry I or L.
Letters
are sometimes missed or transposed but this usually takes the model
so far out of known production period as to make it apparent. (EG HV
1562 on a Sid Patterson only available in 1952/54 and which therefore
cannot be 1941) In three letter groupings, mostly found post 1940
the middle letter, often S or W, may have been used to distinguish
Super Vitesse and Wasp models from similar framesets as the code is
most often found on these models. Different letters and numbers
sometimes found on both seat lug and rear dropout and in this case it
is usually the latter which bears the correct one.
Tandems
1927/1958
As
Sun were major suppliers of tandem fittings to both large and small
manufacturers tandems which show only number codes without original
transfers and letter codes without original badges are rarely
actually Suns.
1959
onwards.
It
is not yet certain whether the code restarted from A in 1959/60 but
some Z and B codes, albeit with three letters, are found on models
only in production late 1959/62. This gives us the following codes:
A64/65 B65/66 C66/67 D67/68 E68/69 F69/70
G70/71 H71/72 J72/73 K73/74 L74/75 M75/76
N76/77 O79/80 P78/79 P78/79 Q79/80 R80/81 S81/82
T82/83 U83/84 V84/85 W85/86 X86/87
Following
careful matching with catalogues, transfer styles and specifications
no models have yet been found which do not fit this dating list.
However some Worksop Suns adopted long number only codes (EG 156202 -
32270) which are at present still mysteries.
Later
Worksop frame numbers are more often found on the bottom bracket
shell or at the top of the seat tube, sometimes with the year code
letter as a separate offset stamping.
...during refinish. A Sun. Lovely vintage machine eventually.