http://www.ro-man.com/shop/index.php?route=product/product&path=395_400&product_id=4448This may help you.
I would email ro-man and ask what pressure gauge they supply.
The picture of the PF one doesn't look like 160psi gauge. Also don't believe it needs to be glycerine filled as you wouldn't have any vibration. Glycerine gauges were normally supplied for air compressors and plant machinery.
The old ro-man r/o I had used a 2.5" back mounted 1/4" threaded gauge. they drilled a slightly bigger hole through the filter mounting plate and used a 1/4" female JG fitting to 1/4" tube to hold it in place. They ran a length of 1/4" tube and joined it into the carbon block to membrane tube with a 1/4" JG equal tee.