This is a Brompton Mk2, built until 2000. If it had a five gear it was an L5 model in it's original state. The brakes have been upgraded to the dual pivot model on the front - standard since 2000 and same as the ones until 2017 (just changed for 2018) - and an enhanced single pivot on the rear, used as standard from 2000 - ~ mid 2007 (then was changed to dual pivot as well). A common upgrade at that time as the older brakes are a nightmare and your's are still ok today, especially in combination with good brake shoes.

Brake levers are still worth upgrading to the current model, the model since 2013 before the current or 3rd party levers. It seems to have a 44t or 46t chainwheel (lower gearing option, useful on the 5-speed), standard for the 3 speed (as well as the 5 speed on the MK2) would have been 50t. The stock roller wheels have been replaced with easy wheels (not yet available at the time the bike was built).

Possibly the bars have been replaced as well - please check the shape as the old ones were affected by some breakages and there was a recall in 2002 for bikes made until 1999. Check here if you's is still affected. Grips and marathon tires are (relatively recent) retrofit as well. Left folding pedal was a paid extra on the MK2 (standard since the Mk3). Rear mudflap did not exist on the MK2 but was standard since the MK3 (2000) on. So either it has been retrofitted or the rear blade has been replaced at some point in time.

For the rear-frame clip there is a retrofit option available from Brompton.
Brompton Q part code: QRFCLIP-RETKIT
Brompton part number: 9040179

Here's a link: https://brilliantbikes.co.uk/brompto...-retrokit.html

Regarding the exact age: Tell us the frame number (on top of the frame at the rear-frame hinge. Then it is possible to date it very ecactly.