Since there is an interest, I would think this is a time to concern ourselves with the way the gauge is mounted.
The LaPav gauge shown is only available in bottom mounting. There is also an E-91 pressure gauge with a back mount. The cost of those gauges is about $40-$50. McMaster-Carr
HERE has gauges that are available with the mounting connection on the bottom or the back. That means the gauges are vertical or, horizontal like the Olympia gauge. Search "Pressure gauges" on their site. 2% accuracy is about .05 Bar. The gauge should be 30PSI with the center 14.7 psi being equal to one bar. I would recommend a 2½" diameter.
Since there is no way to know which way the boiler neck is threaded, each gauge will, of necessity, be put in and adjusted by the buyer.
The horizontal gauge may be better in that it may be read however the cap is oriented. Either gauge may be directionally oriented by tightening with trial and error amounts of Teflon tape in the connection. There should be about one rotation available with the ¼ NPT taper threads of the gauges.
Best financial choice would be I make the adapters with a ¼"NPT threaded mounting hole and you individually buy the gauge that you wish. I can furnish an adapter from that to the LaPav gauge threads for $5 extra.
Thanks all!
Richard
EDIT: I could also add to all, a square ¾" shank to ¼" to ½" from the top for later use as a base for a knob of wood or something else.