British Guiana Stamp Identification

I am hoping someone with better resources could help me identify the stamp below. The watermark is the crown and block CA (wmk 2) however it is not listed in my Scott (2009). I am presuming it is a variety not listed in that catalogue.

looks like an SG#84a

The stamp appeared in various shades and with various watermarks:

1890-91: sea-green w Crown CA
1900-07: grey-green w Crown CA
1905-07: grey-green w Multiple Crown CA
1907-10: blue-green w Multiple Crown CA

SG numbers 213, 233, 240 and 252 respectively, but my catalogue is an older one.

Thanks for your help GeoffHa but the stamp appears to have 2 colours to it, a light green with a darker green for the 'POSTAGE & REVENUE", the value box, and the shading of the value. My Scott only indicates a two tone 1 cent single crown block CA as lilac and gray. This particular stamp isn't even close to that description. Kuhli, can you give me the description of the SG#84a with regards to colours and such. Thanks both of you for your help.

v.

SG84a doesn't exist. SG 84 is a 48 cents carmine-rose of a completely different design.

These stamps were printed using two types of green ink, one of which was more fugitive than the other to identify attempts to remove cancellations for illegal re-use. In the unsoaked state, the two greens appear identical, but once soaked, the main design becomes washed out giving the two-tone appearance.

Thanks peterh for that information. That explanation certainly makes sense. Do you or others have a reference I could do further reading on this subject as my curiosity is up that issuing agents would go to these lengths to ensure the stamps were not reused later. Thanks again to all the responses.

v.

All still valid except for SG 252, which has been abolished and relisted as SG 240c. (SG 240b is the grey-green on chalk-surfaced paper.)