Likely June or July 1967 dispatch date. My G15CS is less than 200 numbers higher and it was shipped in July.
G15CSR had a handful of differences from the CS - usually a larger, chrome tank, low bars, swept back exhaust (Domiracer style), chrome fenders, rearsets and a reverse cam plate in the transmission with the gear numbers reversed on the brow on the cover. CSRs also had thinner spokes on the rear wheel - no need for the heavy spokes of the CS, as the CSR was intended for road use only.
CSR was a variant of the G15, G12, G11, G9 and even the G2, along with the AJS versions
Matchless came up with the letters around 1950 to differentiate the different models - 'S' stood for Sprung, as in swingarm frame rather than rigid; 'C' stood for Competition, usually with a smaller tank, lighter fenders, different seat, and folding footpegs; 'CS' stood for Competition/Sprung, and 'CSR' stood for Competition/Sprung/Road, but was usually referred to as the 'Coffee Shop Racer'
Equipment varied from year to year, with some of the last ones even shipped with 12" rise bars and a stepped seat - referred to as the 'apehanger' CSR
As I stated earlier, the G15CSR/33CSR changed year-to-year. This model was even sold as an Atlas 750SS. The most correct look for your bike would be as they were dispatched in mid to late 1967.
This bike is listed as a 1967 G15CSR, and may be pretty much what yours should look like
Mid-year 1967, beginning around serial 121000, the 750 motor came with distributor ignition and Amal concentrics. The last of the CSRs came this way, and the very late CSRs also came with the CS-style fuel tank - some with chrome sides, some all candy red.
At various times, CSRs came with: chrome headlight, chrome headlight ears, chrome fork shrouds, chrome chain guard, heavy chrome fenders, large chrome fuel tank, low bars, swept-back exhaust. Most of these items varied year-to-year.
As far as I know, all CSRs had the rearsets with the reverse camplate in the transmission and the shift lever facing backward (gear numbers reversed on cover, too)