Not a lot here on these but they date back to the early 70's and until the early 80's. Introduced as a bare bones simple accurate frequency meter (long before sub $20 delivered and evilbay). This one cost me a touch under $50 delivered in an Auction.
This one has the fairly common LED display issues and looking at the manual carefully reveals at least 6 different LED combinations were used all HP branded. The units aluminium case gets fairly warm which wouldn't help.
Apart from the display issue it is reading within good tolerance against my 34401a from 1k-100kHz at low frequencies below 10hz it really falls over which is as per it's spec. Upper end to 30MHz aligns with what signal generator I currently have (better stuff on the way). Where I got it for was higher frequencies where the 34401a and my handheld meters won't go and 80Mhz is plenty as I don't do any RF.
The manual for any of the 5381a, 5382a and 3a makes for some good reading on how 'proper' electronics was done back in the day (IC's are date coded late 1973)
So worth keeping an eye out on evilbay, shipping will be more than the cost of the unit.
Mods coming up for mine:
1MHz OCXO (units timebase) internal oscillator using a divided 3.2Mhz Polytron. Not in a rush for this as the unit is stable and with an IR gun the current XO is running over 35+ degrees so the stability is good.
Fix LCD display.
Use for the next 4 decades
Opening the case was a good/bad surprise. The LED plastic baggy and paper around heat and sensitive bits are not a good combo. Bonus there is one LED of two needed for the display.