Pulse Electronics acquired the Larsen Antenna company/brand name several years ago.
This is the Larsen/Pulse web page for products, repair parts, and supporthttp://www.pulseelectronics.com/products/antennas
Larsen made a number of mobile vehicle antennas from 1/4-wave, to 1/2-wave and 5/8-wave designs.
The mountings varied (NMO, UHF, 5/16-24 or Larsen Mount, BNC, thru-glass, integrated magnetic mount) depending on model.
NARROW your search, by determining the Mount used.
A 48" whip is common for Larsen VHF Kul-rod 5/8-wave (low profile) loading coil.
I had to DIG through old QST archives to find the JM mount.
It was mentioned as New Product in April 1976 QST, page 33.
The JM-mount only requires a 3/8" hole in vehicle body and uses the 5/16-24 stud (later called, LM for Larsen Mount) for mounting.
The LA/JM 150 was offered for fast installations in one-sided (blind) vehicle installations.The 1970s LA/JM was later known as the Larsen Mount - LM
Larsen Mobile Antenna catalog (PDF), before mergers, 111 pages.
DOWNLOAD and SAVE this copy -- getting hard to find and required for Larsen antenna scavengers!http://www.scoden.com/pdf/mobile.pdf
TESSCO sells and support these non-Motorola (NMO) style mounts in permanent and magnetic versions.https://www.tessco.com/products/productHierarchy.do?subtabId=33094
The LM150 models (B for Black whip; C for chrome whip; and U for unplated stainless steel whip versions)
that used the Larsen 5/16-24 stud for mounting. The NMO150 looks identical, BUT uses the NMO mount!!http://productfinder.pulseeng.com/products/datasheets/Mobile%20136%20to%20174%20MHz.pdf
Larsen Antenna Mounts -- the LMB is the 5/16-24 THDShttp://productfinder.pulseeng.com/products/datasheets/MOUNTS.pdf
Product Catalog of current offeringshttp://www.pulseelectronics.com/download/3802/antennas_g00302pdf