from 1957 to '59, the Lenton Grand Prix did have 4 cogs in the rear, and a Cyclo "suicide" front derailleur.
Yes, all the Lentons except the Grand Prix were club racers with SA hubs and single-ring chainsets.
https://www.sheldonbrown.com/retroraleighs/lenton-kohler.html
The Raleigh Record Ace the previous year introduced Cyclo rear derailleurs to Raleigh, still with a single-ring chainset. The Lenton Grand Prix introduced front derailleurs.
The Lenton Grand Prix was introduced in late 1956, and didn't make the catalog before '58.
Your bike looks most like a '57 Lenton Grand Prix frame that may have been altered by the bike shop.
Also worth noting, no one had ridden a derailleur bike before these arrived on our shores.
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it's certainly possible the bike was delivered as a Marque III, which would also account for the brakes - but in '58 and '59, the only paint colors cataloged were green and yellow.
But all through the years, Raleigh sneaked in colors that weren't listed in catalogs (My silver '75 International was never cataloged).
Everything about your frame matches ''57 - the only color offered on the first Lenton and never cataloged.
BTW, the 'FH" on the s/n is more likely a soft stamp of "RB"
Kurt Kaminer would say that makes it a '61 frame -
http://www.kurtkaminer.com/TH_raleigh_serials.html#55
Though he doesn't actually account for your serial number, because he states all RB serial numbers should also have a suffix letter N (etc.)
When they added lever-operated front derailleurs for 1960, maybe Raleigh took all the remaining single-shifter-boss frames and outfitted them as Marque III.
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Here's my '57 frame, built from bare frame as a custom to look basically correct, but everything underneath is newer, beginning with custom Phil hubs to fit the 115mm OLD with 3/8" axle. I used a single Huret band clamp to get a left-side shifter, with the proper Cyclo shifter on the right-side boss.
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