In photo Intake on top/exhaust below. Nissan guides were mediocre brass and standard on the OD not providing adequate press for re-installation. Proper guide press fit is essential in rebuilds. Losing a guide in older engines is more common depending on how many rebuilds have been performed. These guides are slightly oversize on the OD to provide the correct press fit.
The longer, tapering, angle toward the end of the guide is the intake guide.
To order the intake or exhaust guides separately in (sets of 4) use the following part numbering and pricing.