The Solarforce R5 XPG single mode 3-18V drop in from ITC_shop on EBAY has been on sale recently for as little as $12.90 w/ free shipping.I have been using it in a 504B w/extension.The drop in is outstanding and out-throws the other R5s I own.It is definitely brighter with a two 18650 configuration and is almost too bright for close-up or indoor use.It has a smooth reflector that produces a brighter hot spot than the DX R5s.
The Solarforce is also fractionally brighter with two 18650s - not that you'd notice without a meter. However, the DX one is 5 mode which may be more useful. The Solarforce is probably of very much higher quality though
Lumens 0/30/120 seconds
Solarforce L2m DX R5 dropin High247255215Throw10490 lux at 1 metre
Solarforce L2m Solarforce HV R5 dropin270254233Throw11450 lux at 1 metre